Because that's the way Honda designate NSR250 pistons!
The NSR150 goes A/A, B/B, C/C, but the 250 doesn't. It would certainly be easier if it was the same as the 150, but it's not.
Honda only gave piston size information with the part numbers for the MC18. have a look at the MC18 section of the parts manual for an idea.
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Andy.
NSR-WORLD.COM
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