By the looks of it you have a parts gun some one made up. The trigger guard is off a NRA Comm. from 1971. The made 5001 of them. Your trigger guard looks to be NRA4171. I have NRA2021. Your back strap is another number altogether XXXXXSA? It could be 66360SA. Does you paper work serial number match your SAA you frame's number?
My guess is it is a parts gun some one built. But you could always get a Colt letter and see how it came from the factory to know for sure..