I've been digging through this incredible archive for half a week now and I still can't find anything that references it.

The MC part in MCLC might mean "Mix Core" or "Mix Card" however even going back to ProTools 5 these cards are vastly different beasts that for the most part were slices of a larger brick of cards. All I can say is it does SOMETHING with audio simply because of the presence of the MC56K.