And I think I see how Martz modified/fabricated the new mainspring strut anchor...
(It's shown as the "magazine catch" in breakdown pics but it also holds the hammer strut and mainspring in place)...
Edit: Interesting that one of the carbines uses a Mauser C96 stock, and the closeups of the gripframe show the 'straight' lug (rather than the curved Luger lug) that Martz welded on...Which necessitated the 'magazine catch' modification...