Big Duke and Tankie.......... Fitting the .22"/7.62mm interface block into a new magazine is easier said than done. I have done it on saveral/many occasions when they were the norm. The height of the feed lips is very critical for obvious reasons. Just a tad too high and the rounds thud into the top edge of the chamber. Too low and they won't be picked up. A little bit low and some will feed but on others, the breech block slides over the top round. Dave Lines who used to do them had a little jig that clipped over the front mag lug and sat down onto the rear end.
Another 'problem' was that the rivet holes in the interface block are hidden when it's in the new magazine so you have to rely on accurately copying the OLD magazine or laying the interface block onto the new magazine and drilling through. Anyway, it's a good job that magazines were plentiful as I've ruined a few!
I seem to recall that towards the end of the sub cal kits, you couldn't get replacement magazines or the rivets to fit the inserts into a new magazine. We used brass or nickel tube and spread the ends out
As I said, if I was doing them now I'd accurately position the inner mag and partially fill the big mag with an epoxy. Job done