There is some decent info on all the RS models at http://www2s.biglobe.ne.jp/~h-oka/home/rs/rs_etc.htm

You will need to use google translate or similar, to read in english. (But don't expect a perfect translation!)

Some interesting pics, and it does include some details of changes between each model class, and between year to year of the same model class.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30