MG T-ABC

Cleaning my TA brake backing plates, I'm finding light gray under black.
What is this gray? I wouldn't have thought it was primer. I always thought chassis parts were never primed, just sprayed black. Is this an example of the "Use Morris parts" dictum?

Has anyone else seen gray brake parts?
This car was originally metallic gray, but I don't know if that is relevant.

Clausager says chassis and all chassis and brake parts should be black.
This is what I intended to do, but I just thought I would put up the question.

The only brake parts I've seen that weren't black were brake drums painted body color for motor shows. But never for production. Backing plates would certainly be black. The primer I've seen has always been brown, that I can recall.

Ok if I don't hear anything different, I will assume they were using gray primer in 1937.