5/16, 3/8, 7/16, 1/2? bring a set
Deep socket.
1/4 drive would be easiest but 3/8 probable would work too.
There is no physical attachment from register to heater/ AC box just foam seal.
The hole in the chrome strap is 0.70" dia and 1.5 deep. You DO NOT remove the chrome strap, it is...