Hi and welcome to the Forum.
What you have is a .32-20 Hand Ejector that has been heavily modified. Without the serial number, that we can speculate it was made in the 1922-1929 period. But the barrel has been chopped off and a different front sight has been attached. Also, the stocks are some sort of aftermarket replacement.
I don't know where the best place would be to find the part you need. I can tell you that the gun is a K frame. Others here may direct you to a source.
SWCA #2475, SWHF #318
Last edited by JP@AK; 04-21-2017 at 12:50 PM.