80 (80) - Special Quality?

The type 80 was manufactured by numerous companies and has a variety of constructions.

One such form designed to improved power handling and reliability was one with  corrugated anodes/plates by National Union in 1935.  This had only a short period of issue.  (Stokes. J.W. p 111)

I have a specimen, shown here which bears on the base the words "Special Export Test". 

Is it known what testing was done or whether these were special quality tubes by design? It would seem that this is an early example of "special quality" tubes that were later manufactured or selected by test to meet special (usually military) requirements. 

What other examples of "special quality" are known from the 1920s and 1930s?

Regards,    Roy

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