Model 12 Black Diamond Questions ??|Winchester Shotguns|Forum|Winchester Collector

I’ve just bought a good looking Model 12 12ga. with Ser. No. 319629 which puts it at 1922. It has a Black Diamond butt stock on it. It doesn’t have the Trap located just under the stamping of the Ser. No. So my question is this, is this just a Model 12 which has had a Black Diamond butt stock added on to it ?? I thinking for it to be a true Black Diamond Trap gun it would be stamped with the Ser. No. also ?? Any help would be greatly appreciated. 

Thanks