[pilleya]

Originally Posted by Cothron Theiss

It looks to me like a .375 or even .25" hex bore.

Looks like it is the FTC bolt pattern so probably 6mm round or D bore with 6-32 bolts on a 16mm bolt circle. Most likely compatible with the new AM Nubs: http://www.andymark.com/AndyMark-Nub-p/am-nub.htm