That's a sling swivel for a rifle sling...
Several manufacturers of similar designs:
http://item.brownells.com/categorie...ivels-wmach-screw-frt-studwd-scrw-rr-stud.htm
http://www.chuckhawks.com/blackhawk_lok-down_swivel.htm
Several manufacturers of similar designs:
http://item.brownells.com/categorie...ivels-wmach-screw-frt-studwd-scrw-rr-stud.htm
http://www.chuckhawks.com/blackhawk_lok-down_swivel.htm