Late 71 according to this

https://www.yenko.net/forum/showthread.php?t=130132

But I think the High Performance Center folded much earlier, like in 1968.

Correct, but the dealership had two locations that bounced around for a few years after. The 70 SS396 El Camino i almost bought was a true Dana Chevy delivered vehicle as it had pretty strong docs to back it up. Wasn't a motor transplant just valve covers carb and some other bolt on upgrades.