There are at least 4 revisions of the 645/4500 mags. Originals had medal followers and basepads, Orange, Yellow and newest Black. The Yellow followers came in tubes the had a small indent at the top rear of the tube to help hold the rounds back. They required a special follower to clear this indent reason for the "Only use Yellow follower in this mag" warning on the tubes. The Black followers will fit those tubes but came in a tube with square indents on the feedlips themselfs. They hold the top round slightly lower and back and are the latest version. I have had them all and never had a problem with any of them in my guns, but I do have all the newer black follower mags now.