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The sign accompanying the engine. It reads

Atlas Booster Engine

Each Mercury-Atlas rocket had one Rocketdyne YLR89-NA-7 booster engine with two bell-shaped thrust chambers. RP-1 (kerosene) fuel and liquid oxygen moved within the engine through a labyrinth of pipes, ducts, and valves. A gas generator burned RP-1 and liquid oxygen to make hot gas. The gas propelled two high-speed turbopumps that pushed RP-1 and liquid oxygen into the booster engine's twin thrust chambers. The RP-1 took a detour before meeting the liquid oxygen - it passed first through the thrust chamber walls to cool them. Brought together at last, the fuel and oxygen burned explosively to produce about 267,000 pounds of thrust. The booster thrust chambers swiveled to steer the Mercury-Atlas rocket.

The exact configuration of the the engine system changed several times: In the MA-3 configuration, the booster engines' gas generators and turbopumps were mounted with each thrust chamber (i.e., a self-contained engine), but in the earlier MA-2 and later MA-5 configurations, the two LR-89s shared a single gas generator and the turbopumps were mounted centrally, and so the two thrust chambers were considered a single engine (as was the case with most Soviet engines). The Atlas D used by Project Mercury used the MA-2 engine system, while this particular engine came from an MA-5 system.

See SiloWorld for diagrams depicting the MA-2, MA-3, and MA-5 engine systems.