This is the vernier engine for the Atlas launch vehicle. Two engines of this type were required for vehicle roll and yaw attitude control during the boost phase of the flight, before ballistic trajectory was attained, and for making fine thrust adjustments of the missile's booster engine. The vernier is a small gimballed single-chamber engine of low, fixed thrust. Its propellants were drawn from the Atlas's main propellant tanks.
The vernier was developed by Rocketdyne, the same manufacturer of the main engines. The first successful flight of an Atlas, with its vernier engines, was made in 1957. The engine had a maximum thrust of about 1,000 lbs. This object was donated to the Smithsonian in 1976 by Rocketdyne.