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I once come across a 6K8 which had been branded as a 6A8, but apart from the designation there was no manufacturer marked on the valve.

Caused a lot of confusion - it was in a Pilot set and actually worked in a FC role, although the 6K8 was a triode-hexode rather than the 6A8 heptode the set should have had. The indifferent performance caused me a bit of a headache for a while as the last thing you would suspect was a wrongly branded valve!