Here's a graphic, that makes it a bit clearer.
The first and second carriers have the earlier design, with armour sloping towards the rear (most clear on the first carrier).
The second 1940 'Bren Gun Carrier' has the same armour, sloping towards the rear, but it's partially hidden by ammo boxes, tool boxes and other kit.
The third carrier is the 'universal' with improved 'box like' side armour.
Saxon