OK, I'm just going to admit straight up that this class flummoxed me a bit. In the end, much to my shame, I decided it was because I didn't really understand what "history" was. Sad, I know. But I'm a librarian, so I know my tools, and went straight to the OED to find this:
history: that branch of knowledge which deals with past events, as recorded in writings or otherwise ascertained; the formal record of the past, esp. of human affairs or actions
and while I was there, grabbed this:
auxiliary: subsidiary to the ordinary, additional.
Now we're properly armed. I think my confusion was rooted in the fact that there's a specific meaning for History (capital "H", because it's serious) the discipline that is distinct from history the word used casually by non-professionals. The Auxiliary Sciences of History are things that are little-h historical, but not capital-H Historical. That makes them auxiliary - extra, outside the normal, a supplement.
So what about that pesky "science" word? That's a turn-of-the-century thing too, just like the telephone etiquette bit from last week. A lot of areas of study started calling themselves "science" then, some rightly, some less so. You could just as correctly call things falling in Class C the auxiliary disciplines of history, and in fact, some people do.
Now, as to what falls in Class C: lots of things you might think of as your grandparents' hobbies, to be honest, although legitimate scholars also study them. Old coins (numismatics), family history (genealogy), restoring antiques (archaeology) - all auxiliary sciences of history.
There are some real gems here, whether you're a big-H Historian, a little-h history buff, or just interested the esoteric things people research. One of my favorites falls in subclass CB, the history of civilization. All call numbers falling in CB156 are concerned with "Terrestrial evidence of interplanetary voyages". We have about 8 titles in that call number range in our collection, some with awesome tabloid-headline-style titles like "SPACE-GODS REVEALED".
Subclass CC is Archaeology, seemingly normal, but includes CC200-260: Bells, Campanology, Cowbells. Cowbells. Separate from just "bells". I can't tell you how happy that makes me. First, because it's just fun to think of someone doing academic study of cowbells. And second, because it lets me close with this, the first logical co-location of the Library of Congress Classification System and Will Ferrell's hairy belly.