Hi SAKIJOE, Actually the line you see is a mold flashing line from the mold half's coming apart... or, cutting right to the chase.. a parting line.. I think it is the TRUE indicator of a period commercial mag, versus an armourers spare... I have seen plenty of un-numbered fxo style mags where the parting line was buffed off in prep for a number.. but a very few with the line, flashing, intact... Again, just coming from what I've seen and believe.. not written in stone..
... Best to all, til...lat'r...GT