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The marking for No. 1 tube on the injector end ring, just above the baffle at the 9:00 position on the injector.

Picture 1 of 2.

It appears to be a short, squat "1" with an arrow above it pointing to the injector, presumably because the No. 1 tube is a fuel-up tube. (Recall that fuel flows through the tubes comprising the thrust chamber to provide regenerative cooling; every other fuel is a "down" tube, carrying fuel from the forward end of the engine to the exit plane, where the fuel return manifold then redirects the fuel back up an adjacent "up" tube.)