What does the "1" mean after USN (USN-1) on dog tag?

Dear Ms. Thompson,

Thank you for posting your request on History Hub!

We were unable to determine the meaning of the “1” after “USN” on your father’s dog tag. For more information, please review the Dog Tags topic on the website of the Navy Historical and Heritage Command.

We hope this is helpful.

It's not a "1", it's an "I".  The term USN-I means "U.S. Regular Navy - Inductee"