To be clear, are these the Mullard tubes believed to be lower quality/knock offs?
http://www.ebay.com/itm/130295387885?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
At least from the picture, those ones above look like the genuine 70s-80s CV4003 Box Anode, mentioned by Dogmatrix, Halo Getter, in original boxes with cigar rolls. Those ones have a proper factory code etched into the glass that seems to check out. In the picture it is shown as
R4E2.
I think I have this right:
R = Factory code - R is for Mitcham Factory, England, made by Mullard
4 = year code - 1984
E = month code - May
2 - week code - manufactured in the 2nd week of May
In my limited knowledge, looks okay to me assuming what you see is what you get and they are not fakes with genuine factory codes which would be sly to say the least. I've never seen a cv4003 reissue but I don't think the modern "official|" Russian mullard 12au7 re-issues even pretend to be genuine with codes etc and they seem to come in new blue Mullard branded boxes. So I think you are fine with the ones you linked but that doesn't mean the upscale ones aren't perhaps older and sound different.
If Groovyd can tell us what the factory code is on the two ones he has, we could nail down where/when the better sounding ones come from. The factory code is near the bottom of the tube. There should be two lines - the top one is the part-code and the bottom one is the date code as far as I understand.
As Money4Me says, there is bound to be variation across different years/factories, so it would be cool to fine-tune our mullard habits!
and finally, damn the auto-correct keeping changing Mullard to "Mallard". I don't want a Mallard in my WA7.
Jeb.
