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A white stencil just aft of the throat of the thrust chamber, reading "2120" (visible below the drain lines, behind the white wire, forward of the reinforcing bands on the thrust chamber tubes, and to the left of turbine exhaust duct).

This particular engine's serial number (J-2120) matches the serial number of the thrust chamber. It is apparently not uncommon for the thrust chamber's serial number to differ from that of the overall engine.