The center bearing does indeed have a groove for an o-ring indicating it is MC21/28. I stand corrected (yet again).

But just to be sure, someone could have rebuilt MC18R5K using MC21/28 center bearing right?

I understand the only other difference between MC18R5K or MC21/28 crank is web thickness. And that you can mill 2?mm off an MC21/28 crank to make it fit in a MC18 R5K. Does anyone know what web thickness' dimensions?
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MC21R9N track
MC18R2J track
CBR954
CR250