WBZ was an AM call sign that had TV calls and FM calls added. There was also a repeater in Springfield called WBZA. In the old days call letters were assumed to be for an AM station unless otherwise specified. Later when WBZ-FM (now WMJX) went on you'd still hear WBZ-AM simply called WBZ. Arbitron uses the -AM and -FM for any station that makes it into a book.