Welcome to the forum from South Florida and interesting piece you have there. I'm sure some of the weapons experts on here will see it before long and tell you more about it. You may also want to post this in the weapons and ammo forum.
Welcome From The Heart Of Dixie. Be Sure To Stop By The Weapons & Ammo and Uniforms & Relics Forums. If you could post some photos of the lock plate and any other markings over at the Weapons & Ammo forum we can take a look and see just what you have. In the mean time here is a link that will answer some of your questions.