Can anyone answer a U.S. Navy-related question?

[Soupermom]

I'm hoping someone here can help me. When my DGrandpa passed away last May, I was given one of his dog tags. It isn't the usual dog-tag thought, at least not those that I have seen. It is more Oval in shape and he always had it on his key ring. It is made of heavy (silver) metal, but has bent a little with age. My DG was active in WWII, but was stateside. He was a fireman who trained others to be firemen in Virginia before heading overseas. Anyway, this "tag" has his name: first, middle, last. One on each line.
Next it has a seven digit #, displayed as ***-**-**
Next it says USNR-O
Then T-*-**-P

Can anyone tell me what the numbers (*'s) mean? How about the last two lines? I'm guessing the USNR is U.S. Navy something? Is the O his bloodtype?
What about the T/P stuff?

TIA for any help you can give!

 

[RoyalCanadian]

1st set of numbers = service number
USNR=US Navy Reserve
O=most likely blood type -- although this isn't the standard place for putting this info
T-*-**=date of tetanus shot
P=Protestant

 

[tiggerlover]

Usually the service # is the social security number as that is what is on my DH's tags.

 

[meshmar]

Usually the service # is the social security number as that is what is on my DH's tags.

Using a SSN as service number is fairly new, actually. They changed sometime in the '60s if I recall correctly. Before then it was a number - but not an SSN

 

[tiggerlover]

Using a SSN as service number is fairly new, actually. They changed sometime in the '60s if I recall correctly. Before then it was a number - but not an SSN

I never knew that, thanks.

 

[Soupermom]

THANKS everyone! I thought the P might be protestant, but the other stuff had me lost! Happy Independence Day! And thanks to all those who serve(d)!