the numbers refer to the different cabs.
#11 cab being the earlest. They had problems with maintaining them as it was hard to get to the engine to service them. Main clue to identifying a #11 is a one piece grill and a flat cover on top of the bonnet (hood). The radiator cap was situated externally at the top of the grill. these also had wooden windscreen frames.
#12 cab looked similar to the cab 11 but with a 2 piece grill and a liftup bonnet and the radiator cap was internal..
#13 cab had the headlights in the bodywork and a forward slopeing windscreen.
hope this helps
