FWIW...Full GI-spec magazine feed lips sometimes need adjusting for the correct release point. I've run into it in about 10% of the unused magazines I've bought over the years...and when I got a few of Check-Mate's end run of their last contract, I found the same to be true.
The "Hybrid" style lips provide the later, more gradual release of the GI mag, with a timed release point in the correct place. It's a Colt design, and CMI makes the magazines to Colt's specs. A little more versatile in most guns..and they come pre-tweaked. The price is the same.