That's a mono fader .Notice It only has 3 wires going into it. It is used on AM cars with a single front and single rear speaker.
On the 1970 E body ,a stereo fader would be used with a 5 speaker system , 3 in the dash and 2 in the rear. It would require a diffrent harness as well which would include a crossover box.
Greg


gregward@mchsi.com phone 256-852-0955