6350 tube who makes them?

Looking for info on this tube.
I think this is an NOS only tube; used extensively by Jud Barber at Joule Electra,
NOS type of input/driver tube used with older VTL and Joule among others. I bought a pair from Tube World last year for my older VTL 2.5 pre amp. The Sylvania Black plates are concidered among the best of the 6350's. I think I still have a pair somewhere.
Listed below are the ones I own and use. I know these are still available and work in VTL amps. All are old stock.

RCA black plate
RCA gray plate
Sylvania black plate

Substitutes that work and perform differently:
Telefunken 6463
GE 6463