It is most likely either for a '45 C.I. Flathead or for a Knucklehead. Is there a size stamped on the rim or is that your measurement ? Both models were offered with 18" rear wheels and using the star hub which it appears that you have.
Elijah and Jon, thank you for your detective work. I do not have the wheel in hand yet as it is in the process of being shipped, so I cannot look for an accurate wheel measurement. Looks like mystery solved. Cheers, Wade