The fader control (P/N 7309211) used a knob with a tang on it so it would "grab" the shaft and turn it, resulting in music being directed to the front or rear speaker. I'm not sure what you mean by "not the same" since,if you look in the AIM (UPC U80, sheet A2, item number 2 - Knob, P/N 3954239), is the same as the bass/treble knob (UPC U63/U69, sheet A2, item number 11 - Knob, P/N 3954239). Sounds like someone has modified your radio.
Ed