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'll look for the other numbers in a while. The tube's fermenting in my dac at the moment and sounds rich, detailed, 3-dimensional and quiet. 
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