This is the Thompson Center quick release system for TC Hawken, Pennsylvania Hunter, or White Mountain Carbine. Part numbers are:
No. 9920 - Base Adapter
No. 9975 - Quick Release Base
No. 9995 - Quick Release Ring
I don't have the mounting screws but they should be available from Fox Ridge or any decent gunsmith.
$55 (shipping included) /OK, make it $40
/ for everything. Pictures are below: