An “S” should be present on the cowl or after the registration number on World War Two jeeps. It went on the jeeps that passed the radio interference test whether or not they had a radio. AR 850-5, Change 9, dated 27 Jan 1944, a regulation that reguired an “S” marked on vehicles. This same change also stated an “S” marked vehicle was to be preferred over a non-S marked vehicle when installing a radio. Common sense? Well, sometimes the Army had to spell it out.‘S’ symbol on cowl of a 1942 Ford GPW and Army Jill sitting on the fender.