Jonas,
Following up on Jerry's point, it's my belief this car is quite possibly, numerically, the last one fitted with the round tail lights; this despite the fact that some earlier (by chassis number) cars were fitted with the rectangular items. This came about because AE 65 was ex-works in January 1955, whereas AE/AEX 55-64 all left the factory at least two months later, and I know that some of these had the later lights.
Like Jerry, I believe the same goes for all the other changes - long boot to short boot, optional discs, chrome on brass radiator grill to alloy etc. It appears the changes took place by date, not chassis number, and as above, this is because the cars did not leave the factory in strict chassis number order. To illustrate this point it's worth noting that the last cars to leave the factory in 1957 (20 December) were AEX 221 and BEX 389. Despite the difference in the chassis numbers, both cars would have been finished to the same specification and, for instance, the optional disc brakes could have been fitted to both cars.
Tim