"USNR;mm3" = his rating and branch of service: Machinist's Mate Third Class (abbreviated MM3/c or MM3), United States Naval Reserve (USNR). So you don't have to ask later, during wartime, the Regular Navy, Naval Reserve and the Coast Guard are, for all practical purposes, one and the same. Individuals have essentially the same duties, obligations, priviledges, etc. regardless of component. Likewise, the Regular Army, Army of the United States, Reserve Corps and National Guard.
Bureau of Naval Personnel Liaison Office, Room 5409 9700 Page Avenue St. Louis, Missouri 63132-5100
If he's living you'll need a notarized letter of consent or Limited Power of Attorney. If deceased, Proof of Death (authenticated copy of Death Certificate, notarized affidavit of surviving spouse or member of his immediate family, etc.)
At one time - and not that long ago - they would give you everything down to the number and location of every mole on his gluteus maximus - free of charge. A proliferation of "laws" - and other factors - in recent years has changed this, in some cases dramatically.
When you receive whatever info they send you, contact me and I'll show you how to use it.