Does anyone know a good online reference for WWII US dog tags?

I am looking for information on how to decode the information on a US dogtag from early in WWII. It is the oval type with the officer’s thumbprint acid etched on the back. I think these were only issued prior to the US entering WWII and had been in use since WWI. All of the reference sites I have found seem to skip over this period.
These sites are good, but they don’t have info on decoding the US Navy Reserve format. As near as I can tell, the format is: 1) FIRST NAME 2) MIDDLE NAME 3) LAST NAME 4) RANK & ??? 5) BLOOD TYPE TETNUS SHOT (T-XX-XX) 5) COMPONANT (i.e. USNR).
I am trying to figure out what the letters after RANK stand for. Such as M.C. – assigned to the Marine Corps? Medical Corps? Doctors, Chaplins, and Medics, Nurses and others were assigned to the Marines, but did their dogtags reflect this?