I bought a late 70s era New Haven 600AT (mostly a Mossberg 500A) with a 18.5" barrel. I noticed that the end of the magazine cap was not threaded for a 10-32 sling swivel stud but the lug had a plug screw that was threaded 10-32 in the bottom. I removed the plug and installed a .25" 10-32 stud. Easy-peasy.

It's a "warning" roll-mark barrel. I don't know when they started with the warning barrels or if it was just a replacement barrel. I'm just wondering if some enterprising former owner did it or did some Mossberg barrels came that way at one time.

Anyway I much prefer the bottom of the lug position over the cap position.