I took some pictures on the one on my 76 D100. I believe it to be a factory set up. Others more knowledgeable than me can view and verify if it is ma Mopar or aftermarket.

76D100 SB End Clamp.jpg76D100 SB Frame Clamp-1.jpg76D100 SB Frame Clamp-2.jpg76D100 SB Frame Clamp-3.jpg

2012 Challenger SRT
2012 Charger SXT AWD
69 Charger R/T
78 Chrysler New Yorker 440CI
89 Dodge Daytona ES 2.5L Turbo
99 Ram 2500 4X4 5.9L6