Yup, it's a firing-pin retainer.
These are supposed to be a split pin with a funny head so that you can compress the two sides with a V-slot tool and then push it straight out. You can order the right pin, make up the tool and replace properly, or you can do what I did....... and just show in a straight shank and rivet the ends to upset the thing in position. Works for the short term, but DEFINITELY not collector-spec or milspec!