Looks like an improvised lock/block. In your third pic, with the piece flush with both sides of the hammer it would work in the frame, if slid out, the cut out portion seems like it would ride over the frame and prevent the hammer from moving. The divots on the bottom of the block, the spring you have on the side and the divot in the hammer where the cutout is, maybe would have been a ball detent to make it click.
(just a guess, I did do a couple of google searches, didn't return anything related)