I have a truck that has chrome pull-down door handles. There is also vent windows on both sides. there is a 3-speed shifter on the colum. i think it is a 51, its serial # is 102 22 P 21394
If it has both pull down door handles and vent windows, then it is a 51', or at least the doors are. If the speedo is 80mph, and the driver's side of the cab has no cowl vent, then the whole truck is most likely a 51'. Another good forum to try is at www.stovebolt.com those guys know a lot about pre 72 chebby trucks. Over and out. bullnose