No tapping required, just the correct size hole For the SS/ silver boss.
Get the Anarchy AT RH thumbrest; SKU AI-TR-AT-RH that has the SS/silver threaded boss.
Drill 1x hole in the correct place, RHS chassis and that boss slots not screws right in, is held in place thru the flushcup on the LHS of the chassis, and the screw from the RHS in via the thumbrest. All screws supplied in that kit, not the flushcup. I may have drilled another hole in the LHS for the flushcup, can’t remember if that was already there or not.
The RH thumbrest screw fits into the plastic/ brass threaded, captured nut that sits in the keyed skin mount.
Easy!