Product review (4 stars): Match the part number with your water heater model number.
This replacement tank 91059 only fits the Atwood model GC6AA-10E.
Not sure you allow links, but here's the install manual from Atwood listing parts and instructions for around 60 models.
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See page 39 for choosing the correct replacement tank by part number.
Upper Left box in photo shows model number.  It's a sticker inside the door.