Can O' Worms. ALL imported AK's have the serial number on the trunnion. The ATF has consistently ruled for years that the receiver consists of the trunnion and the receiver as riveted together. The two cannot be separated without destruction of the receiver. I think the ATF issued an erroneous opinion letter to Wes at Soupbowl Enterprises a few years ago. That's what this whole " the trunnion is different on the shotgun than the rifle" thing is all about. Every other AK has the sheet metal receiver as the trunnion but for some reason the Shotgun was ruled differently. My ass. Ive got an opinion letter off to the ATF tech branch to get a ruling on this because I want to build an AOW with a flat. My guess is they will affirm that the sheet metal part is the receiver and not the trunnion. It will be my ruling. It will apply to me and my case just like Wes's ruling applied to him but an entire industry has been built from that erroneous letter.