I'd thought of that too Strangely. You can see the different markings on the sides of cab doors and tank turrets, such as a yellow triangle, white square and red circle etc etc..
But here's another for you. Look carefully at the end of your No4T and Bren chests and you might find, if they're not painted over by now, 4 little nail holes that form the corners of a rectangle 4" x 1.5". These are the nail holes that hold or held onto the end of the chest a wrapped 2.5" x 5" tinplate label holder with fold-over sides. This was used to hold the packing note or transit label that was slipped into it. These little things could be used a few times until the little fold-over tabs broke off. The holder was then pulled off (usually with a BIG screwdriver or pair of pliers that ripped big divets out of the wood) and you'd nail another transit note holder on..... and so on.
So, if you have some of these spaced nail/tack yoles together with some lumps of wood chiselled out, this is the reason