That is like the middle one of pic#2 in post #22. The one in that pic has a yellow follower,but the rest of the mag is same as yours' pictured.
I believe/assuming/guessing that the yellow follower was a quick design change to replace the red followers for the indented mags with a design change to subsequent black followers. Could be the injection molding process is less expensive for the black design compared to the yellow design.
Your mag pic shows a pretty deep indentation,wonder if it's too deep and the follower 'hangs' up on it.