Back in the 1980's we used to TRADE 30rd mags to civilian FFL's for old 20's because the 30's craped out in the field alot.
In the early to mid 80's there were a bad batch of mags made by Cooper Industries. All of the Coopers I have seen appeared to be made well, but they had bad springs. The springs were so weak, and I am not talking used, but fresh out of the wrapper, they an empty mag the follower would not even actuate the bolt catch. When shooting, the first 22-23 rounds would feed well, then it would start giving bolt-over-base misfeeds. I experimented with wood spacers under the spring, which compressed the spring and reduced the capacity to 20 rnds, the mags ran perfectly. This proved to me the springs were weak. Cooper lost their contract, their mags were taken out of service and dumped on the civilian market in the late 80's. The follower was not the problem, the weak springs was the problem.
Sometimes D.L.Sanchez mags, the other contractor in that period, had a few problems during full auto fire. The green follower was made to solve this problem.
Shooting semi, I have not had any problems with black followers, but when I refurb my own mags, I usually replace them with greens.
Regarding the "England" mags, I got mine from CMMG, who supplied them with green followers and chrome-silicon springs. They work perfectly. I did not shoot them with the original followers or springs.