This is an Apollo Reaction Control System (RCS) rocket motor, or thruster. The RCS maintained the attitude of the Apollo Service Module and helped it to maneuver in space, undertake minor midcourse velocity corrections, abort separations, and make emergency attitude maneuvers. It produced 100 pounds of thrust and could be fired in spurts or steady thrusts as needed. RCS thrusters were mounted in groups of four in boxes called "quads."
RCS thrusters were used throughout the Apollo program from 1968 to 1972, on Skylab 1 in 1973, and in the Apollo-Soyuz Test Project mission in 1975. This object was donated to the Smithsonian in 1986 by The Marquardt Corp.