Simon,
There are several differences over the 4 years after the aero screen version, but they also got "corrupted" on major overhauls or rebuilds, and it would have gone through at least on of tohse. No consideration was given to Contract number, year of build etc. If you have a chassis number, that is the best way, then you can find out if it has the correct dashboard, (2 types), roof cupola, etc.
The chassis number could be on a plate in the cab, also there may be a large dat plate on the chassis under pass side, with post war census number, chassis number and dates of rebuilds. Adjacent to the plate will be the chassis number stamped in side of chassis. The round rad badge would seem like an early one as later, around 42-43 they had a rectangular pressed metal badge with only "Bedford" script logo on it.