Bottom mounts from the underside of the chassis through a 1" hole. Diameter of round portion of saddle measures .981". I measured 1.30" screw mounting centers. Black phenolic body, nickel plated saddle and very secure contacts. More (four!) dang grounding lugs on the underside of the saddle than anybody would ever need. Why? This socket does not look exactly like anything Fender used in the '50s-'60s but it is a nearly exact fit replacement for some old Fender amps and is thus highly regarded by many esteemed amp repair artisans. Be warned that this is one of the tightest sockets we sell. I could not find a new old stock USA tube that could not be fitted to it but some imported tubes with non-standard size pins may not work. Set of six sockets.