The Advance Design doors are interchangeable from '47 thru '53.. so you could take doors off a '51 and they'd bolt on a '47, which is probably what happened in your case.
It happens quite frequently.. either a previous owner preferred having vent windows, or the original doors were shot and they just found a set of '51 doors and put those on.
'47 thru '50 trucks didn't have any vent windows, and had turn down door handles.
'51 was the first year for vent windows, and retained the turn down door handle of previous years.
'52 was the first year for the 'push button' door handle.
