The 1947 & very early 1948 Advance Design trucks had the gas tank mounted under the bed & the filler neck came out through the bed side.There were no filler necks on the cab for those years.
Late 1948 - 1955 first series trucks had the gas tank mounted inside of the cab behind the seat . Not sure if the bed mounted filler necks used the same cap as the cab mounted tanks but.... all of the cab mounted filler necks {regardless of tonnage} for late '48- 55 first series were identical & the caps were the same.
