Finn,

NSR engines share the same engine numbers. They start with MC16. This includes the NSR250R models MC16, MC18, MC21 and MC28.

The MC16 had MC18 had hexagon shaped cylinders (outside) while the MC21/MC28 had rounded/smooth shaped cylinders.

MC16, MC18 and early model MC21's had no lubrication hole for the right side main bearing in the engine case allowing the oil pump to feed oil. Later model MC21's and MC28's did. From there it's mostly about the electronics.