I have a Model 500, 20 gauge with 18.5 inch barrel, vented hand guard and pistol grip. I want to add a sling. The take down screw is threaded for a stud but I cannot find any stud advertised for this model that will swivel. I’d rather not have the stud screwed tight to the take down screw as tension on the sling may cause the take down screw to loosen. Does anyone offer a swivel type stud that will screw into the take down screw? Also, this particular shotgun is marked: Mossberg Made in USA North Haven, Conn. 500 20GA. It is not identified as a 500C. Interesting. Serial No- V0127*** I appreciate any advice. Thanks.