This appears to be a prototype chamber for a 2,000 pound thrust MG 19 rocket motor developed by Reaction Motors, Inc. for use as a JATO (Jet-Assisted-Take-Off) unit for heavily-loaded PBM Navy seaplanes. The MG19 was developed in 1943.
However, the Navy did not adopt liquid propellant JATOs and only used them on an experimental basis. Both the Navy and U.S. Army Air Corps opted for simpler and cheaper solid propellant JATOs. This object was donated to the Smithsonian by the Reaction Motors Division, Thiokol Chemical Corporation in 1985.
Reaction Motors Div., Thiokol Chemical Corp.