I remember reading something about that, I think there were some experiments done with that in europe, but not in the US AFAIK. What was being used in the US was undoubtedly DNP.
That's partly true, DNP was the first formula sold in the US under the brand name Formula 281. However, it was changed after about a year and rebranded as "Improved Formula 281" and that version contained DNC.
This was actually the main reason it was removed from the market, it was misbranded because the FDA deemed label claims to be inaccurate and/or misleading. It was never banned outright, the FDA had to request the discontinuation of the product and, the lab making it (Isabella Laboratories), complied. It was still a pharmaceutical wild west back then, misbranding was a valid concern... Today it's pretty much par for the course.
Also noteworthy, DNC increases metabolic rate approximately 10x more per milligram than DNP... And THAT was what was being sold and eventually removed from market.
See:
https//fdanj.nlm.nih.gov/catalog/fdnj28339
