Geezagillard PM'ed me with a link to this thread:

http://www.nsr250.net/forums/viewtopic.php?p=47573&sid=91accea606a76d7ab950041b5af9fcd9

which states the following:

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

However, this has massive overlap between models and doesn't even mention MC18-120xxxx numbers at all. So where does the MC18-120xxxx designation come from? Anybody?
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...