Last 6 years ive been drawing SB stuff, I've always called everything as class, be them 156 strong boat class or just single unit. Its a vessel of its class, be them sole presentative of it or not. I've have no idea why MConrads before me done it that way, Propably becouse most naval publications list all warship classes as class, despite the class strength... or he just wanted to give the pedant nitpickers something concentrate on. There are bunch of lurkers in this forum who tend to have taken habbit to post solely on critisising indifferent aspects of these drawings, and it would be awfull if they wouldn't have anything to say, wouldn't it
Anyways, more seriously:
As for Pr.68U-1 and Pr.68U-2, there's no burden to grasp the concept of single ship class idea, since in Soviet practice, classes of ships were always destinated with rather detailed project number system, and if the ship varied considerably from the standard, it was made own class of ship with different Project Number. both Pr.68U-1 and Pr.68U-2 are more closer to their actuall mother class than they are towards each others. Therefore it would be simply false to try to catechorise them under same class between themselves, and more apparently so to be listed under regular Pr.68bis class since the differences are quite idionymic.