Help ! Mullard 12AU7 production codes

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I came across production codes which seemed unusual to me (based on info I have pulled off the net). The tubes are Mullard 12AU7s (shields, small halo and medium/long plates) with the codes
Gf Gf
2K 21 (or maybe 2l)
While I understand Gf to be a 12AU7 (and "new /revised construction"?), is the "2" the factory code (which would be Philips Eindhoven - the printing is too poor to confirm country of origin)? If so, this would mean the K and 1 are the month codes? And if so, does this suggest that the tubes are pre 1957?
Or is the "2" the year code and that therefore the tubes do not have a factory code?
I also understand that not all Philips tubes had change/batch codes? Is this true?
Any help would be appreciated!
Tubenana

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Kimber Kable

Topic - Help ! Mullard 12AU7 production codes - Tubenana 23:52:40 07/31/06 (0)

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