Yep , it's either a 3 speaker , or It's a 4 speaker on the 70 B body .( The C and E bodies had 3 or 5 . )
I just went thru all this last month when we did the 8 track for a 70 Charger R/T SE sub lime Six pack car . I wish I had taken pics of all the harnesses together. it was not at all like the usual 3 and 5 speaker harnesses configurations that I am used to seeing.

It was the standard 3 speaker harness with crossover box, then the second harness actually plugged in to the radio, the 3 speaker harness plugged in to this harness , as well as the rear fader control ,
and of course the plug for the harness going to the package tray for the single rear speaker .

Greg