Maybe for a commercial version of the M-1 Carbine? Otherwise, I don't know. I do know some of the commercial "non-G.I." carbines had parts that were not exactly like the military version. Unfortunatly I do not recall all the exact specifics.
It is for a .22lr carbine. I have a .22lr M1 carbine made by Johnson-Tucker in the early 60's. They used alot of GI parts. The trigger group is USGI except for the hammer which looks like the one you have.