Nickel plated steel Jazzmaster/Jaguar Tremolo arm sleeve. The sleeve (A.K.A. "bushing" or "arm receptacle") is the gizmo that bolts to the bottom of the whammy assembly and sticks up through the top of the plate to accept the vibrato bar. This bushing was designed in the '50s to mate with the 'pop-in' Fender USA Vintage Jazzmaster Jaguar Tremolo Arm Bar.