My April 66 427/425 Coupe has a Water Pump (Part # 3856284 with a casting date well matched to to the car so I assume it original) with a "flange" type backing plate secured with large slotted pan head screws. In other words, the backing plate edges flare out slightly rather then completely flat. The judging manual states a flat backing plate. Curious if the difference may be production related - ie where two backing plate types common on the 66 Big Blocks while the JM picked up one through a smaller universe of car observations. Of course my pump may have been rebuilt and a relacement type plate could be in evidence.