I believe this is due to the wheelbase difference between Challenger and Barracuda convertibles. Challengers are 2 inches longer. Challengers use one,and Barracudas use the other to clip the backseat in.

70 Charger R/T SE 472 Hemi 70 Charger R/T convertible 70 Charger R/T V Code Sixpack 69 Charger R/T SE Sunroofcar 68 Charger 383 68 Charger 318 71 Charger R/T 70 Challenger convertible 71 Challenger convertible 71 Cuda 340 09 Challenger R/T Classic