Had to do some research, but what I found was Keystone was making bumpers before AMD offered them. The Keystone bumpers are brought in raw and require the final stamping (around the license plate area) to be done in Wisconsin. The AMD bumpers come completely done in chrome from Taiwan.
The AMD set I had for my Challenger were not very good. The first time I tried Keystone I lined them up with an original and they felt as thick and the same weight. With the AMD bumpers I could feel they were lighter right away. Next time I get a set of Keystone bumpers in I'll put mics on them and get a real number.
It's possible that they are both getting them stamped by the same company but with different spec metal. I'm still looking in to this. But for my cars, I will stick to original re-chromed or Keystone.
I tried a set of AMD Challenger bumpers and they were junk. Took them right off car the same day. Could not stand how thin they were and the chrome wasn't good at all.
I've had really good luck with brand new Keystone Show Chrome bumpers. They are not the flimsy thin metal like the AMD. Pretty sure David at Roseville has some really good prices (better then advertised) on Keystone bumpers if you tell him you're from Moparts (or tell him you know me). After re-choming some originals in the past, I just get new Keystones now and they are super nice. Here's the link to his site...
http://www.rosevillemoparts.com/product_...roducts_id/8508Not sure if Keystone makes a 71 Charger bumper though. Might be worth a call to David.
Keystones are AMD bumpers, rechromed....