Looks like someone was a victim of the plug falling out. It's an easy fix.I had one that was done by a tiny (like 1/8 inch) hole being drilled in the bottom of the mag, and the plug spot welded on from the outside. It broke off after firing abotu 100 rounds. Apparently welding a heavy plug onto a piece of sheet metal is not the appropriate way lol.
I drilled and tapped the plug, filled the plug with epoxy and screwed it on TIGHT, then ground the screw head off so you couldn't get a screwdriver into it. Took abotu 5 mins to fix and there was no question of legality.