Gents,
Don't rely on the chassis/frame numbers - it's the crankcase and cylinder numbers that are representative of the year of manufacture and build. This will also amount to how many bikes were built in each production run.

As`Les has stated, it was the end of the NF5 manufacturing run so HRC cleaned out the parts bin in making the last of the 1992 models using any and all available parts. The NX5 model was completely different as you are all aware.

RS250RE XXXXX and RS250RFXXXX is common across the range. RS250RE leans more towards a 1991 manufacture but is also on some 1992 models just to confuse the matter.

PipeTurkey and I have been researching this for years and have a substantial data-base of case, cylinder and chassis numbers. If you'd like to supply your numbers - it will help our database - and you in the long run in helping identifying your bike

Cheers,
Steve S