Steve, I finally managed to take some pictures to explain the detachable draw system. The system is not complicated by any means, and would be something readily achievable with manual machinery (although I would be tempted to modify it somewhat).

Here are some pictures of a draw out of the action to show the profile and slots which mate with corresponding races in the action body. Every craftsman develops their own method with things, but my approach with fitting these is to initally harden the draw and temper right back (steel is EN9) to allow easy filing. These draws are retained simply by a tight friction fit in the action body, and a degree of hardness helps to maintain a tight fit to the action as the part is tapped in and out during fitting. This is fine for shotguns but I would want to use some form of retaining pin if this were used on a double rifle.

The draw is the last thing to be fitted when actioning a gun, and it is preferable to have dialed in the desired amount of push and the barrels must be jointed down to the breech face. The following pic shows how the draw mates to the corresponding surface on the rear barrel lump.

I begin fitting the draw using good old lamp smoke, but once I get close to bringing the gun back on the face I will reharden the draw and temper back to a light straw. I will then switch to using a permanent marker, which is much thinnner than lamp soot and gives a clearer indication of the bearing. The high spots are taken down with diamond files until the gun is jointed back down. It must be noted that the draw is in effect attempting to hold the barrels off the face and care needs to be taken to ensure good engagment with the draw (you can feel the barrels "picking up" on the draw if you close the barrels slowly and feel for the slight drag) but it cannot be excessive or it will bring the gun off the face, even an imperceptible amount will lead to accelerated wear of the gun. The position of the top lever should be noted with the draw out of the gun, and the top lever should return to this same position with the draw fitted...if the top lever is held off then the draw is bearing too hard.

And that is it! A simple system, but care must be taken in its fitting. I hope this was of interest.