Tim -
The number is on the outside of the rear case that houses the finned water element, and it faces the firewall. (obviously, I don't know the correct name of the "finned water element."
I shot these pics before I rehabbed my heater:
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Wider view of number in my previous post. The front cover of the heater is still attached in this photo. Visable thought the round opening.
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Full view of the case with number. Only the "54" shows in the photo, a couple of inches right of the red arrow. The front cover was removed in this photo. This photo is higher resolution so that you can move it around and see the details. Hold "Cntl" and tap "+" to enlarge.
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Heater partially disassembled.Let me know if you need more pics to determine the location of the number.