David: Plating is a "build up" operation, unlike bluing and case hardening, and if the piece is to be plated all over, a size allowance has to be made for the plating or the parts with close tolerances won't it together. I would guess that the lock plates were plate while attached to the frame for this reason. Plating, like electrolysis to remove rust is a "line of sight" process, so that the interior of the gun and locks would receive a very thin plate if any. Chrome plating is sometimes used to improve the fit of shafts to things that the shafts have to fit.