I'm not the low cab expert, no idea what GM did then, but I'd assume there were some 37 models that had external fill necks, and ones without external necks used the same tank but would need to have the fill neck/cap added when the tank was replaced, either from the old tank or a new neck ordered separately .... what you see in the Master Parts manuals is the applications the parts applied to and what was available in the day, search in it and see if the neck is listed .... so to your original question, the tank itself from 37-46 will fit under the seat, and your tank is no newer than 46 - someone must have cut the hole in the cab to accept that filler?
Bill
