Go to http://www.brownells.com
At the top of the page there are tabs. Click on the one for Schematics. You want Schematic ID# 683. The part number is 133 on the Schematic. Make sure you do not also need part number 136 also. It is the retaining ring.
133 60405 067C60405 301-000-125 Sling Ring, M1 Magazine Cap, $12.36 retail, $9.89 wholesale.
136 60409 143B60409 301-000-184 Sling Swivel Retaining Ring $10.35 retail, $8.28 wholesale.
Good luck.
DGA