That's an oddball size and why I didn't mention it. The most common are the 200's and 400's.There is a third size, 333K. All are the same dimensions.
Is the Heater Natural Gas or Propane?
Try and find other labels as suggested above.
If you can't then post pics of the gas line into the heater and measure the outside diameter of the pipes. The specs in the chart below are inside diameters and we can figure out what you are piped for from the OD.
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