The only difference between the 300A and 300B is the locating pin, which is not used by any modern amp. I believe the 300A was only made by Western Electric for one or two years, then replaced by the 300B.
I would check the provenance of any tube claiming Western Electric heritage very carefully. WE never made that many, and very few were sold to the public. (WE didn’t sell equipment, they leased it to theater owners.) No amplifier maker in the Fifties, Sixties, or Seventies manufactured amplifiers using 300B’s.
Modern reproductions look very close to historic 300 series tubes. Be cautious.