Anyone happen to know what the screw size of the lock screw for the mainspring retaining sleeve? The one I bought from Numrich is M2.2 x 0.45 and it is too small. A M2.5 x 0.45 won’t start. Not having any luck with imperial screws either. Thanks in advance!

Try BA (British Association). 8BA is very close to 2.2mm.
I've worked on several Martinis with BA threads.

Thanks for the reply. I found another breech block and the M2.2 screw fit it. I got suspicious that the first breech block might have a partially stripped hole, so I turned a slightly larger diameter screw and that seemed to catch what remains of the threads. I think the block is too hard to tap to a larger screw diameter. Kind of scratching my head on how to best fix this issue.

Surely that screw doesn't need to be tighter than you can achieve with what you have done now and a drop of blue Loctite?

LOL. You are probably correct