Hmm. I'm thinking that factory-installed (i.e. "original") units may all indeed have been black, but that replacement units, or ones added as 'optional accessories', may have been plated. Is this a possible explanation for the pattern of units we've seen so far?
My thinking is that perhaps those original factory production line servos were purchased from Girling in large bulk shipments, and that a deal may have been struck to purchase them cheaper by ordering them simply painted all black instead of plated? After all, if by avoiding the cost of plating they could get Girling to cut them a break of £1 or £2 per unit, that could add up to some serious and much-needed pounds, for what we now know to have been a factory struggling badly to make a profit.
Whereas ones for resale (i.e. 'accessories') may have been 'standard issue' plated Girling units, ordered in limited numbers as needed by Rootes for spares stocks (i.e. not 'bulk shipped'), and which were normally only offered by Girling in plated format to improve their appearance/shelf appeal?
I know for a fact that the ones I've pulled from original cars and have rebuilt in the past have never had plated end plates... indeed, I usually pop them into my blast cabinet and dust off the old paint and rust before repainting them, and there's never been a hint of cadmium or any other plating on any of them. So I know at least all the ones I've seen have never been plated, and I'm pretty sure they were all factory-installed units.
