Production engine from the Mars Viking spacecraft program used for the first landing on Mars… 45 years ago.
Constructed from Beryllium, Columbium, and stainless steel by North American Rockwell/Rocketdyne, the engine provided propulsion to take the Mars-bound orbiter with its attached lander to Mars and an orbital insertion around the planet. The engine was produced by Rocketdyne for the Jet Propulsion Laboratory (JPL) and includes gimbal attachments which allowed the engine to be adjusted on a rotational axis for in-flight course corrections. The beryllium thrust chamber was derived from the Minuteman ballistic missile program.
“As close to a flown engine from the project as you will find.” — C|Net. More info and photos below. Part of the FV Space Collection.
NASA sent two Viking spacecraft to Mars in the summer of 1976, and each comprised of an orbiter, which photographed the surface, and a lander, which studied the surface and conducted several experiments. The whole spacecraft orbited the planet for approximately one month, using the images relayed back to mission control to identify a landing site.
The Viking 1 landing on Mars was originally planned for July 4, 1976, the United States Bicentennial, but imaging of the primary landing site showed it was too rough for a safe landing. The landing was delayed until a safer site was found, and took place instead on July 20, 1976 the seventh anniversary of the Apollo 11 Moon landing. Viking 2 soft landed two months later.
Closeup of MOOG Bipropellant Valve, S/N 15
More details from Spaceaholic: this bipropellant engine is fed by a hypergolic mixture of Nitrogen Tetroxide & Monomethylhydrazine (N204/MMH); these hypergolics increased engine reliability as the constituent propellants ignited on contact when applied through the injector plate into the thrust chamber, eliminating the requirement for a separate ignition source. 
The engine was utilized to provide midcourse trajectory corrections while the Viking was enroute to Mars and executed the orbital insertion and orbit trim maneuvers of the Orbiter/Lander spacecraft upon arrival at the red planet. Orbital insertion of Viking 1 required a long engine burn-38 minutes of thrust, which consumed 1063 kilograms of propellant, slowing the spacecraft from its initial approach speed of 14400 kilometers per hour (8948 MPH) to 10400 kilometers per hour (6462 MPH). To bring the spacecraft to the proper point at periapsis (1511 Kilometers/939 miles above the planets surface), the spacecraft was placed in a long, looping 42.6-hour revolution of the planet, reaching first periapsis; orbital apoapsis was ultimately trimmed to 32800 Kilometers (20,381 Miles above the Martian surface).
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