Well ..... TA DA ...... NO. Nada clue. Normally I don't answer unless I can contribute something. ...... Ah .. anything. Were it I on the
job, I'd find something close in the form of a Cap Head Set Screw. Drill out the old threads, Counter bore for the head of the new screw,
thread the hole and call it done. Or, even a close size long enough set screw.
So long as the actual Main Spring retaining screw is intact almost anything can be drilled and tapped to work. Unfortunately, it has been my experience over the years, when Uberti makes a change, they only support the "old" parts until they run out or have had no call for
them and dispose of them. For Uberti rifles that "ancient" Uberti doesn't support ANY replacement parts unless they are the same as
new build.
Coffinmaker