IIRC, that comes up in KRAD's Articles of the Federation.
Aha! Yep! And there were three
prototypes, known to Juliana, created either before or after the B-4.
After the revelations of "Nemesis", I told Jeff Lang that if he ever did a sequel to "Immortal Coil", perhaps the prototypes would be B-1, B-2 and B-3.