As I previously said part #is not in 70-71 parts book. I know 72 Cuda brackets are different from 71. If you have the bumper, see if the face holes line up. Back when things were being sold many items such as bumpers were sold as fits from 70-74, when in fact that wasn't always the case, and I found out when trying to install the parts.
That explains why you can't try to see if they work. I'm not familiar with the rubber bumpers or how there attached. Hopefully those Brackets aren't for them.
I looked at my brackets and the # I have is 2962-284, which was the pass. side bracket. This is on a 71 Cuda. I couldn't see any other #'s without tearing the front apart. That part that I have also fits a Challenger. It seems that 70 & 71 use the same part.
One long arm has no markings. The rest are as follows....
2962315
2962280
2962281
2962285
can't read my scribble on the last one....LOL I believe it's the same as the one you saw. Ends with 284.