I've been thinking about this one for awhile now, and here's the only two things that I could come up with, that make sense to me. Either that's the motherboard model, or it's some kind of a denotation by the manufacturer of whether or not it supports Preshott's. Now, obviously, the major manufacturers at that time didn't have a model 478b, but that doesn't mean that someone like PCChips, or an OEM manufacturer for Dell or E-machines or the like didn't.
And sure, a motherboard that supports Preshott's should really be a 478E, but I can see them also using B, as in "the first would have been A, if we had known that we were going to have to build Skt. 478 boards with much better caps, for a later revision chip", so we'll call this one the B version. Of course, none of that has anything to do with heatsinks.:laugh: The same heatsink will fit on all Skt. 478 boards.