I will post this again... seems appropriate.
Three kinds of jeeps were used by the Navy.
1. Contract jeeps, i.e. MB-NOM, MB-NOM-12, MB-NAVY-MZ-1, MB-NAVY-MZ-2, MB-NB. To date it appears contract documents have yet to be found on Navy jeeps. We know they were produced, but in most cases not how many.
There was a Navy order for Ford GP's, a 1/2 dozen or so........ Painted the same as the one USMC GP..........Lusterless Olive Drab.
2. Jeeps under contract with the Army that were diverted under some kind of agreement with the Navy.
This includes 50 MA http://g503.com/forums/viewtopic.php?t=19081
653 GPW made in 1942 under QMC contract 11424 and 1310 MB in 1945 under ORD 33-019-3339.
So far the only difference between a standard jeep and these diverted jeeps may be in lieu of a USA number at the factory, these jeeps may have had USN marked on them or unmarked at all.
3. Jeeps obtained by the Navy/USMC by whatever means from the Army, used as Navy jeeps.
The Navy sometime used inventory tags, dash tags, to keep track of jeeps on a base, but this may be post war, or maybe for stateside jeeps.
So, how to tell if your jeep was first used by the Navy. It is a case by case issue. Certain features were used on Navy jeeps, hooks and rings. Obviously if your jeep was painted forest green or one of the Navy greys, then it may have been used by the Navy. Dash tags and other similar radio equipment tags may still be on the jeep. Navy or USMC numbers on the hood or above and below the side grab bars may be under layers of paint.
After looking at the current serial numbers on G503 I would venture to guess that the 1310 jeeps fit somewhere between MB 458700 and 460010. These are jeeps as noted under 2 above.
Navy jeeps when marked as such seem to have registration (serial) numbers from 5000+/- (revised) at the start of the war and 300000 at the end.
USMC jeeps seem to be marked 37000+/- to around 110000.
I want to add that I agree with Mike, the USA number was blacked out and the USN number applied... leaving the "S" as is.
As for the USMC driving Navy jeeps, the USMC was a Department of the Navy. The official logo decals of the time period reflect that, though I have a number of jeep photos with the decal that just shows the United States Marine Corps on the top without the Navy notation.