Sounds like dynamat but then all of them look pretty much the same (sound deadener).
Yes, the resonators were I believe to reduce some of the "boom" because it echos through the trunk. Many guys used to remove the mufflers and leave the resonators only, thats the way my 70 Challenger is (440 6 pack).
70 Challenger RT 440-6 EFI, 73 Cuda 416-6 EFI
05 Hemi Durango, 01 Ram 4x4, 14 Ram 2500 4X4, 10 PCP Challenger 6 spd RT, 01 Viper GTS ACR, 52 B3B w/330 Desoto Hemi, 70 Hemi RR (under const)
9 x Challengers. AAR Cuda, 4 RR, 2 GTX, 4 Chargers, etc... (too many to list)