Hello everyone: rpm, Black Diamond guns were last cateloged in 1938 and were still available on Special order until 1948. I agree with gunut ! a Md 12 with just a matted barrel and black Diamonds would be rare. It could be a stamped with Trap, Skeet or none of those. A Solid Rib, is the most common in the smaller guages, but it could have a "2 Pin" vent rib with the Duck Bill receiver Ramp, the receiver would be non-grooved, and matted like the rib. Black Diamond grade guns were honed, the typical machine marks, in the slot for the action slide bar were removed, as well as the back side of the action bar, along with the back side of the bolt, and carrier. Depending on the vintage the bolt and carrier, it may or may not be jeweled or engine turned. Also the "Black Diamonds" in the wrist of the stocks lined up almost perfectly, meaning the points top and bottom were in line as well as the front and back tips. I too have seen "fake diamonds, they are distorted compared to originals. The bottom of the butt stock regarless if it's a pistol grip or straight pull will have a slight curve or "belly" from the back side of the pistol grip to the toe, or where the wrist area, ends to the toe on a straight pull. At least all the ones I've seen have. Short of removing the trigger guard assy. and the bolt, to look for machine marks. I can't add much more. Let us know what you found out. If possible take some digital pictures, and post them, for review. A 16 ga. Black diamond is not as common as the 12ga or 20 ga. So if it looks good and the price is right, it could be a good investment. Regards Dave