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The digits "148" painted on the forward end of the thrust chamber.

Information stamped into the thrust chamber identifies this TC as "Unit Number J-2148," and an ID plate appears to be affixed over a '2', so the ID plate matches the painted-on thrust chamber number.

Note the ID plate at left. This means that the "148" digits are just counter-clockwise around the thrust chamber from the ID plate, putting the digits more-or-less at the top, the way thrust chamber sits on its handler.