This is American Rocket Society (ARS) Test Stand Stand No. 2 used in ground tests of experimental rocket motors. Built in 1938, it had a thrust capacity of 200 pounds.
The stand notably proved the effectiveness of James H. Wyld's regeneratively-cooled motor in tests from 1938 to 1941. This led Wyld and three other ARS members to form Reaction Motors, Inc. (RMI) in 1941, America's first commercial liquid-fuel rocket company which later built the 6000-pound thrust rocket engine for the Bell X-1 research rocket aircraft that broke the sound barrier in October 1947.
The stand was last used in 1942 when loaned to RM. It then was loaned in 1953 to the ARS. It was donated to the Smithsonian in 1967 by the Reaction Motors Division, Thiokol Chemical Corp.