That (replacement) cock is narrow in the breast and neck compared to earlier Germanic cocks; altogether less robust looking than the cock on the Marshall rifle (which was repaired by brazing and maybe the engraving was lost in heating and possibly re-straightening). Look again at the Marshall #41 in Shumway's RCA volume 1.
Maybe this lock from Jack Haugh (corrected- Jack Brooks) could work? Seems about right in period and general shape and styling, except this has a blind sear screw. Could make it go through.
Engrave it, change the frizzen spring, and you might be good to go
Well, hush my mouth!!! Am I ever embarrassed! I was digging aroung in the shop an what did I discover? I have that set of lock castings from Jack Brooks just waitin' to be assembled. 'Course I did'nt know what I had till this post. Jeez.
Love it when a plan comes together...
Ez