@45000CUP on the upper edge. Maybe some bolt lapping involved to get the guide lug bearing and he wouldn't use the overheated receivers.  I have a cracked one around here somewhere, in the 13000+ serial # range.  Nice paperweight that was once attached to a rifle that became a parts guns once I found the crack.  After the Roosevelt match this weekend, I'm going to get a machinist working on the barrel I need for the 6mm Niemiller Rimmed.