OK, so if I just assume that the MC18-1012815 is stamped on the headstock/steering head part of the frame, then the model of NSR250 you are looking at is obviously an MC18.
There are 4 models of MC18:
1988 r2j
1988 R4j (SP version of the R2j) Has gold magtek rims, Rothmans livery
1989 R5k
1989 R6k (SP version of the R5k) Has gold magtek rims, adjustable brass screw in top of front forks, remote reservoir on rear shock, dry clutch, and "TERRA Racing" White/silver livery.

The R2/4j has tacho redline at 11,500rpm and runs a PGM-I (or just "PGM")
The R5/6k has tacho redline at 12,000rpm, runs a PGM-II, had high mounted mufflers.

Hope that helps.
Perhaps study it up here

Also, for what its worth, the MC16/18's have hexagonal shaped barrels/heads, the MC21/28 has round barrels/heads.
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