Thanks very much for that. As stated, my barrel-less B grade 16 ga. has these recesses, but much longer in size. It is a four digit SN gun, 102X as I recall. I have never seen barrels with anything on the barrel lug that would mate or correspond with this milled out area. When you refer to it as a draw lug, are you saying additional lugs on the sides of the barrel lug are present to draw or cam the barrel set rearward into the standing breech?
This has complicated my search for a set of barrels and forearm that I can fit to my gun to make a "shooter". I found a really nice set of Baker Damascus 16 ga. with original 26" length, but they do not have anything that would engage with these milled recesses. I am assuming they are originally 26" because they touch at muzzles, have normal choke constriction, and are stamped 26 on each barrel and the forend loop.
