Up until 1978... there were WA's, and WB's, and some WD's, but no WC's? Gettysburg avoiding the phonetics for "Water Closet"? :-)
WC#xxx was reserved for RACES and issued to local Civil Defense stations operated by various non-federal governmental agencies.
Very few were issued, and the FCC just jumped over that series when WBs ran out.
I vaguely remember the Hamilton County Ohio Civil Defense EOC having the WC Callsign posted on the wall.
I could be wrong, but I think it also was also used on the PATWAS nets, a sort of precursor to SHARES that had some larger state, and local CD EOCs inter communicating with federal units.