Hi Jacques, welcome to the forum.

The NSR250 has 3 barrel sizes with 3 matching piston sizes:
A barrel <--> B piston
B barrel <--> Blank piston (no mark)
C barrel <--> D piston

So it is correct that your 'B' barrel has a piston with no mark on it.

The left picture sure looks like an MC18 barrel to me. Although I cannot confirm 2/4j or 5/6k
I have no real idea what the right picture is of, but suspect that it is either MC16 or MC11. I'm sure someone else will identify it for you.

For what it is worth, the Honda manual states that the piston to bore clearance should be 0.035mm to 0.044mm with a service limit of 0.08mm, and that the piston OD is to be measured 15mm up from the bottom of the skirt on standard Honda pistons when working this out.

Also, this may be of some interest to you,..
I have blogged my restoration of an NSR250SP MC18 R6k.
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