This is the XLR35-RM-1, the liquid fuel rocket engine that powered the MX-774 test missile, a predecessor of the Atlas intercontinental ballistic missile. The engine produced 8,000 pounds of thrust from four chambers and was also gimballed. This meant it could be steered by the swivelling of each of the four chambers. The propellants were liquid oxygen and alcohol.
Three successful flights of the MX-774 were made at White Sands Proving Grounds, New Mexico, in 1948. No further flights were made, but the engine demonstrated the effectiveness of gimballing and also proved the missile's monocoque airframe, both features later incorporated into the Atlas. This object was donated to the Smithsonian by the Reaction Motors Division of the Thiokol Chemical Corporation in 1985.
Reaction Motors Division, Thiokol Chemical Corp.