[Arran]

I don't know about how far back this goes but Phillips, Valvo, and Mullard were all divisions of Phillips at one time. Mullard was connected with Phillips going back to the middle 30s at least, after the war Phillips took them over outright. That's one of the points I find so amusing about the audiophools, they will pay top price for something like an ECC83/12AX7 just because it says "Mullard" on it but ignore identical tubes made in the Phillips plant in Holland, or the Valvo versions made in Germany. I don't think that the quality control would have varied that much between plants by the late 50s to to make one tube type made in Britain better then one made in Germany or Holland withing the same company.
Getting back to the metal spray coating on Rogers tube types, sometimes the black pain will peel off for whatever reason but I have never seen one where the shielding itself was coming off. Even on ones from farm battery sets from Alberta, that may have been stored in temperatures that were well below freezing. Whatever process Rogers came up with to make it bond worked.
Regards
Arran