Here are a couple of photos of the centers that show the thickness of them and the ribbing you mention.
I assume this heavy one is for the 24" weights as the curvature does not fit on a standard 28" center properly. It bottoms out against the center before it bottoms against the stud nuts.
I bought a pair of heavy 28" weights a few years ago that had these mis matched plates.
I got a pair of the correct ones from Jim UT in a trade.
I still have these mis matched ones here if anyone ever needed one or both of them.