Well I went ahead and ordered four with springs. I got them to replace the M-261 .22 adapters in four mags I got in a box of stuff. Turns out I didn't need them after all as the .22 adapters drop in on top of the original 5.56 follower.
Anyway, they are in fact made of aluminum (or some type of zinc alloy) and the quality looks pretty good compared to an original Colt aluminum follower. I went ahead and installed one along with a new spring to see how it fit. Dropped right in and moves up and down like it's supposed to. I'll update this thread when I take it to the range as a previous poster said he had feeding issues with his.
Shown next to original Colt follower, the dark one on the left:
Mag with after market follower on the far right:
Adapters I was replacing: