"U" is USB.
There seems to be a pattern to the models
DxK ssabcnnh
x B-> Colour (B for Bayer?) M->Mono
ss 21->640x480, 31->1024x768, 41->1280x960
a A->no trigger, B->trigger
b U->USB, F->firewire, G->gig ethernet
c if not present->CCD, if C->CMOS
nn ???
h only possible if field "a" is B, then .H->I/O