This foam looks like insulation pieces that I believe are standard on on all C3's (at least early C3's). They are known as "Pad - Wheel House Deadner." There's two of them on each side. The PN''s are 3947939 and 3956231. (1970 part numbers) The wheel house referred to is the rear wheel house.
The AIM drawing to look at is Sheet B9, UPC 1.
The factory side exhaust (I'm pretty sure) were offerred in 1969 only. The factory side exhausts didn't have cut outs in the rear quarter panels and rear valance for the factory sidepipes. Have a looks at these areas to see if there's fiberlass repair work. If there's fiberglass repair work, the car came with standard exhaust. If the car came with standard exhaust and was back fitted for sidepipes, there will be repair fiberglass in the rear quarter panel cut out (i.e. hole). The standard rear valance had a a cut out also that matched the rear quarter panel cut out. If standard, there will be fiberglass repair to close out this opening....UNLESS a rear valance for a factory sidepipe was substituted, in which case there'll be no repair work.
Also, for the factory sidepipes a little bit on the body fiberglass had to be cut away under the door sills. I forget this detail, I think the bit of fiberglass is right behind the front wheel opening. Have a look for some possible bubba activity.
If the car came with factory sidepipes and was back fitted for standard exhaust, look at the muffler hangers and other hardware. Does it look like factory hangers or generic hangers from a muffler repair shop. Here you probably need to look at the AIM to see what the factory hardware looks like.
