I had a huge Carvin folded horn 2X15 in 1975 that used two 15" CTS speakers. I believe they were rated as 130 watt speakers. I tried a pair of EVSRO 15's in that cabinet but the CTS sounded far better.
By the time the B15m came out, the CTS speakers used in the 70's were gone. The M may stand for "MagnaLab" speakers which were either CTS or Eminence. They looked different, with solid aluminum dust caps instead of the vented aluminum of the earlier CTS.
They were pretty good speakers for the day, on a par with what PV used early on (pre Black Widow.) I used an old PV with the square magnet speakers with my GK1001RB for a few gigs a few years ago, and the speakers sounded and performed great. I usually used EV or Black Widow cabinets with that amp. There is a character to those old CTS and Eminence speakers that I rather like.