Sam, many cars before than had fillers directly on the back, but
mostly still on the right, towards the back - name some trucks that had the filler front or rear, or even drivers side before 49 - sure not GM - name one truck of any kind post 50's that has the filler on the pass side - people stopping for gas at those old stations stopped on the street if it was on their side, and pulled in if they were going the other way, filler still right side, toward the pump - if the filler was in the back or front, it still wasn't on the "street side"
the lock thing is as I said - went w/ the filler, and the reason there was only 1 is cuz yer truck was just about the cheapest vehicle made, and they didn't waste money on frills - some farmer or tradesman in his overalls didn't care - cars had key locks on both sides, [but not on the back doors] and if they could have gotten away w/ making the ladies in their dresses slide across [split seats mostly] they sure would have!
that's my story and I'm stickin to it!

if you've got a better one .... what was yer question?
Bill