Mitchum or Blackburn for the Mullard 6922?

Do Mitchum production and Blackburn production sound different in the Mullard 6922? I believe I once read somewhere that Mitchum made tubes for the military, whereas Blackbutn did not, so, presumably, with better materials and/or tighter tolerances or testing. 
i believe all E88CC/6922/CV2492/CV2493 were made at Mitcham factory (code R). Blackburn (code B) made ECC88, though having the Master facility as well. I never traced a Blackburn made E88CC even to back 1961 made ones, if ever made at that factory is very rare and prior 1960.
Mitcham ones after 70 have different construction and sound different.
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Just bought a Blackburn e88cc on Ebay. I'll provide feedback by and by, though without a Mitchum to compare.