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I'm posting this on behalf of a friend who did this work for me. It's his gift, not mine.

Having done several reproductions of the Lee Metford/Enfield Cavalry Carbine magazine, I found accurately marking cut lines on the donor magazine and holding the new floor plate in place during brazing to be two of the greatest challenges. This jig greatly simplified both operations.

The jig, with some un-shown attachments, was developed for the Cavalry Carbine magazine project. It works equally well for the Lee Speed magazine project. It's uses are twofold. It provides a stable base for creating the templates and marking the donor magazine and it holds the floor plate securely in place, and prevents warping, during brazing.

The upright section is at a right angle to the base and has a vertical groove that is a slip-fit for, and a little deeper than, the raised spine on the magazine. The raised section on the base provides a place for the narrow top front section and U-shaped top rear section of the inverted magazine to index on. When in place, the back of the magazine fits tightly against the back of the jig while the front and rear top sections index on the raised base. The back of the jig and front of the magazine are the primary points of contact. The trapezoidal top part with the screws in it is used to hold the new floor plate in place during brazing, and will not bee seen again until a little later in the tutorial.

The magazine to be copied is mounted on the jig in order to make the templates. That is easier if the follower and follower spring are removed, but it can also be done with them in place - so long as the magazine is held firmly in place. Ensure that the back of the magazine is in firm top-to-bottom contact with the upright and the front of the magazine is in contact with the raised base.

The front and side templates could likely be made of any number of materials. I prefer relatively thin but stiff aluminium sheet stock because it is relatively easy to work. The blank for the side template needs to be right angled at the lower rear corner, have a width greater than the front-to-rear measurement of the magazine (excluding the spine) and a height a little greater than that of the magazine. Lay a piece of sheet metal equal in thickness to your intended floor plate material on the jig and hold the template next to the side of the magazine while indexing against the back and bottom of the jig. The reason for the sheet metal is to adjust the template for the thickness of the floor plate - which attaches to the bottom of the magazine body.

Using a sharp scriber, carefully scribe the template along the floor and front of the magazine. It helps to darken the template with a felt marker or layout dye.

Carefully cut and file to the scribed lines.

The front template is notched for a close fit to the raised section of the base. With a piece of sheet metal between it and the base, hold it against the front of the magazine, scribe and cut to the line.

The rear template is a little more challenging. Mine is made of a 0.465" (approx. 15/32") thick piece of aluminium. The width is equal to the width of the magazine at the rear. The slot, which is a slip-fit for, and slightly deeper than, the spine indexes the template on the rear of the magazine. The rounded lower portion indexes on the rounded upper rear area of the magazine. The upper section has been thinned to make it easier to work by hand. With the template held in place, carefully scribe along the back of the magazine. The spine extends through the floor plate and the notch is there to avoid inadvertently scribing across the spine and cutting it too short. You don't need to slip a shim between the template and rounded section of the magazine when making the template, but you will have to either slip in a shim or hold the template a tad low when using it.

Time for a little woodwork
The mould for the floor plate is carved out of a dense and tight grained piece of hardwood. Maple works well. Oak does not. Start by tracing the front-to-rear curvature of the edge of the floor plate onto the block and accurately cutting the block to match. Draw a centreline on the block,and also lines indicating where the front and rear of the floor plate will be. Hold the original magazine in place and trace the sides. Alternately, one could measure the width of the magazine at the front and rear, mark the block and join the lines. Now the fun begins. Carve out between the lines to precisely match the profile of the floor plate. Avoid inletting it below the magazine depth at the edges. Gouges work well to start, finishing with scrapers. Use a transfer medium such as Prussian Blue and continue cutting and fitting until a good even transfer and smooth finish are achieved. This is a tedious process, but the quality of the new floor plate depends on the accuracy of the mould. Power tools tend to be an excellent way to quickly get into a world of trouble, and are not recommended.

FINALLY - time to start the conversion

It is my understanding that all BSA Lee Speed fish belly magazines were conversions of SMLE No 1 ten round magazines with the movable front right-side feed lip. Although any ten round magazine that fits an SMLE can be converted, I consider Long Lee magazines and those with movable lips to be too scarce to convert and try to limit conversion to later SMLE magazines.

Having removed the follower and spring and placed the shell on the jig, carefully scribe lines on each side and across the front. Do not place a piece of sheet steel under the templates. Take the magazine out of the jig and while holding the rear template in place, scribe cut lines across the rear.

There are likely several ways to cut the magazine. I favour a Dremel with a cut off wheel. Firmly mount the magazine and cut outside the lines with your weapon of choice. The edges of the finished magazine body slope upward toward the front-to-rear centreline at an angle of about 17 degrees. If the initial cut is too close to the line, there will be gaps between the floor plate and the sides at the ribs. Note the cuts at the rear. The spine will be dressed shorter later in the process. The spring clip at the front can be removed either before or after the cuts are made. I find it easier to do it after. Do not dispose of the cut-off bottom section. It will come in handy a little later.

Where you go from here depends on your comfort level. I use the bench grinder to grind the sides almost to the lines while maintaining an angle well in excess of 17 degrees. The ends are best left to hand tools. The sides need to be dressed to the line at an angle of about 17 degrees. I find it easiest to put the magazine in a padded vise at a 17 degree angle and file horizontally. The ends are filed at angles matching the slope of the adjacent sides. Leave the spine standing proud, but file the excess sheet steel off the sides.

Now grind and file the part that was cut off to exactly match the bottom profile. It will be used to hold the floor plate in place during the brazing.

The new floor plate is made of un-plated soft sheet steel and the material I am using is 0.031" thick, which is approximately 21 gauge. I am using that because it is what I have on hand. Twenty two gauge, which is 0.036" thick, would likely be easier to find. Make a tracing of the original floor plate on cardboard about the weight of a file folder and cut it out about 1/8"oversize on all sides. This is your pattern for cutting out the blank for the floor plate. Cut a notch the width of the spine centred on the back of the pattern and stop your cut at what will be the edge of the finished floor plate. Trace the pattern onto the steel and cut it out. The width of the rectangular piece of steel, which is slightly inlet into the mould, is the same as that of the spine. It helps hold the sheet metal in place during shaping, and does not extend into the cavity of the mould.

I probably should have started by removing the paint
To shape the floor plate, tap lightly with a ball peen hammer. Many light taps are better than a few heavy blows and the ball must be nicely rounded and smooth. Any imperfections will be transferred to the floor plate, so start by taking the time to dress out any flats or imperfections and smooth the ball. Although the contour of a large hammer best matches the shape we want to achieve, I find a very small hammer gives better control. I steady the front of the floor plate with my fingers while pushing to the rear. Try to avoid moving the front from side to side. The fully shaped floor plate will closely match the mould, but will likely be covered in numerous very shallow humps and hollows. That is not a problem since they will eventually be filed out.

Assembly time
Check the fit of the floor plate to the magazine body. When you have a good fit, file the notch just a little deeper if needed to achieve a snug fit on the spine. Place the magazine body on the jig with a spacer in the slot to move it forward so the floor plate can fully engage the spine. A little can be trimmed off the back of the floor plate, if necessary. Having cleaned all surfaces to be brazed, clamp everything down and you are ready to braze.

The two sides and the front can be brazed while it is in the jig. It needs to be taken out of the jig in order to braze the rear.

I wish I were better at brazing, but it is what it is. I need to work on using less filler. File and sand all the edges flush and file the spine extension flush with the floor plate. Very lightly file the humps and valleys out of the floor plate and sand down to about 240 or 320 grit. Drill the drain holes and you are done.

Comments and suggestions are welcomed.

Unfortunately, none of the photos of the process will show up when copied onto here. Should anyone be able to provide assistance in having the photos posted here, I would greatly appreciate it.