One correction to the last posting. In the last couple years, B-C converted to dBx encoding at a time when they believed there still was a market. The first release was a release of previously released Dolby B encoded tapes. After that, their tapes were only done in dBx, not both. You don't see these very often has only a few were released and they soon stopped production and closed.
A few tapes were done as a unidirectional tape, but these, too, were encoded and, not really a different format as they, too, were 4 track.