Data codes on M-O valves
"Regarding the M-OV (e.g., GEC) date codes, I'll refer you to that "Best Year/Style for KT66" thread on the Marshall Board which I think is still up, I tried to explain my thoughts on that there. In brief, it appears M-OV started using letter code in the '40s for year, changing each year until they ran out with "Z" in 1968, then they switched to YYWW in '69, through the '70s and into the '80s. This may not be perfect, i.e., they may have kept or dropped a letter somewhere along the way in the '40s or '50s, but it seems to hold up pretty well. So on those KT77s with "ZD Z", the first "Z" (on the left) is the year, 1968. "D" is presumably April, just like on old Mullard codes, although some '50s M-OV tubes have a second letter well above "L" in the alphabet, so that doesn't seem to fit those. Don't know of '60s exceptions, though. The other "Z" (on the right) is for Hammersmith, London factory, and was used in that position at least since the beginning of the '60s. Before that, one or two-digit number codes appeared there, but I don't know how to decode those. I do know that M-OV had several factories outside of London in addition to Hammersmith (because the latter was a visible strategic bombing target of the Germans, and had several "near misses" right alongside, although the factory was never hit), some of which lasted into the '50s (but none after '58, I think), so the diff number codes might reflect that. Or maybe something else, I've never been able to confirm that KT66s were made outside of Hammersmith even when there were other factories set up.
Incidentally, the very last audio tube made by GEC at Hammersmith is known to be a particular KT88 Gold Lion with code "8617 Z" (they had some kind of commemoration), and other audio tube production had stopped by then, so if you get a GEC with a date code later than this it's definitely fake! I've seen lots of KT66s with 1982 dates, but that's about the latest I remember.
Regarding the British military codes, e.g. "Z" for Hammersmith M-OV, "AD" for Brimar, etc., these are definitely different from the factory codes used by Philips for Mullard, Valvo, Amperex, etc. I think that, on non-military valves, M-OV tended to use them from the '50s on but Philips didn't. They appear on the British military versions, however, including Philips-made as described, e.g. after "KB" designation, for example "KB AD" on Brimar box plate CV4004s, "KB/D" on Mullard CV4004s, "KB/Z" on military CV KT66s, etc. I've also seen "L" and "ZD" on M-OV military KT66s, so maybe these are the other factories, Perivale, etc. I was referring to. I believe the British codes are posted on the web, maybe on the Triode Electronics site. I know they've got the Philips factory and type codes listed there. "
From a post from Jeff W. on Vintage Amps Forum