Page 2 on the MC16 MC18 Parts guide lists engine & frame numbers. I expect the same detail will be in other parts guides.

It would be great to add this info to the specs pages

Model___________ Engine no range________ Frame no range
--------------------- ------------------------------ ----------------------------
NSR250RG (MC16) MC16E-1000047~1021049 MC16-1000033~1020966
NSR250R2J ______ MC16E-1031032~1058121 MC18-1000020~1026996
NSR250R4J ______ MC16E-1039597~1051877 MC18-1008557~1020796
NSR250R5K _____ MC16E-1130033~1149562 MC18-1100017~1119489
NSR250R5K-II ___ MC16E-1130033~1149562 MC18-1100017~1119489
NSR250R6K _____ MC16E-1130054~1148151 MC18-1100038~1118089
NSR250R6K-II ___ MC16E-1130054~1148151 MC18-1100038~1118089