This is the injector head of the Apollo Service Module Propulsion System. The engine steered the Apollo Service Module (SM) towards the Moon, placed it in lunar orbit, and after completion of the mission, returned the SM to earth. Developed by the Aerojet General Corporation in the early 1960s, the engine was non-throttable, gimbaled (steerable), and ablatively-cooled. Using storable propellants, it produced a thrust of 21,900 pounds and was first deployed on the flight of the Saturn V unmanned Apollo 4 mission on November 9, 1967.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.