The first issue, on the side where the transfer sits are 2 small lugs to line the transfer up and very early versions (or maybe preproduction ) had these lugs missing (1, 1a, 1b). Thanks to Bob Tutt of the UK, I am able to show you one of these very rare versions, but this model also has casting changes to the rear passenger step as well, being thinner than the normal issue so a very early version indeed. It like the following, had what is called by collectors as '4 over 4' which relates to the side windows (2). This was changed to strengthen this area to '8 over 4' with the addition of a small brace in the upper window (3). 

The underside of the top deck can be either bare metal or tan the same as the top.

The drivers came in light blue (4), dark blue and black and there is really no significance to which is on each model. All wheels were 'grenodised' (bare metal) until the last run which had black finished wheels (5). Axles can also be crimped or riveted.

An interesting point is that the D1 box on this model is one of the very hard to obtain and fetches a high premium when sold.