Foolishly I looked at the mc28 page, the 18 and 21 are correct.

The (mc28) pic labels part 9 as the snap ring, part 10 as the piston clip and has no part 11 but the text says;

9 91301-KV3-771 O ring 60.7x1 1 1 1 1 1 1
10 94560-68200 Ring Snap 68MM 1 1 1 1 1 1
11 94601-15000 Clip Piston pin 15MM 4 4 4 4 4 4

So, quick glace, part 10 (piston clip) part number is 94560-68200 which is what I ordered.
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