You have a TC quick release mount there.. Thompson Center offered these for folks who wanted to be able to remove a scope, or swich out a scope, without removing the base iteself... you are supposed to be able to closely maintain zero when you re-install your scope. Never tried one to see how well that feature works, i found that when i had one mounted i never took the scope off, so it was a little redundant to have a quick-release mont i never took off the gun.. I had one for a while of a non-braked .35 Rem and it seemed to hold up with rattling apart, my only complaint was it was HEAVY, about twice the weight of a conventional set of rings/bases by leupod & probably 3 times the weight of a weaver base with the proper rings.