Hello Ben,
The makers plates had the factory address's removed after dunkirk. Most vehicles had the big brass plates pre-dunkirk, post dunkirk the bedfords had a smaller plate with no address on, or as you say the larger plates had the address machined off.
The plates i have seen with the address milled or ground off, were done at the factory, post dunlirk, judging by the type of machining used, and the age of the vehicles i've seen them on. The vauxhall factory was heavliy bombed in august 1940, and quite simpily we had given them the address to do it. I'm not sure if other manufacturers suffered the same.......
Sorry back to carriers now
John