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A reaction control system (RCS) thruster. Four such RCS thrusters were clustered together to form a "quad", and four quads were arranged around the lunar module's ascent stage to provide attitude control

The accompanying sign reads

Reaction control system rocket engine.

Donor: Armand Small

The ID plate running along the bottom of the valve assembly (the black and silver ban just below the two cylindrical valves) reads

Engine - Rocket, LM
Part No.
Se [goes around the corner; "Serial No."???] GAEC No.
GAEC Ser. No.
Cont. No.

None of the information seems to be filled in.

Note that "GAEC" stands for "Grumman Aircraft Engineering Company," the name by which Grumman was known at the time.