Sir, I am impressed to say the least. I have tried braising a floor plate back on to a cut down mag, it was an exercise in frustration.

The sheet metal parts fit well enough in the cold state but started moving and distorting when I applied enough heat to melt the brass.
So i tried tack braising first, which then let go and poing! when I tried to fill the gaps. Doh!

I got it finished but had a lot of clean up to do, and the seam was kinda wonky. It was functional but did not look like it was factory, as yours does!

Practice makes perfect, I have done but the one!

I am totally amazed by the products turned out by Victorian tin smiths.
They could probably join two pieces of foil together!

Obviously it can be done, you just did it but have skills and techniques that I need to learn.

Does your floor plate fit inside the body profile like the original? How to stop the thing warping under heat?

I have been thinking about making an oversize floor plate with a margin around the edges.
Cut the mag so the plate will butt up flat around the perimeter.
Bind the plate in place well with iron wire.
Flood the inside with silver solder. Then clean up the outside to remove the margin.

Am I getting close? Probably not.

Not much on the net other than hunters cutting down the mag on a band saw and mig welding a flat plate back on to make a flush mount unit..

If we ask nicely, would you show us how it is done the next time that you do one, please?