I've been playing a lot with the early A classes, then bought several Bs with Resonance, then played some of C. That resulted in a ship unlock message, but I never got to see which ship is that. I'd attach a pic but it doesn't upload for some reason.
Anyway, it looks like an empty rectangle without the lock, but still saying "unknown vessel" and not reacting to cursor. And you can't buy with Resonance in class C.
Since then I returned and completely finished A classes, i.e. won with every ship, but this void ship (pun intended) still didn't appear. Tried to win again with adjacent one, the Explorator C, it also didn't help.
So as of now I'm missing three more ships in C class that I can't unlock.