Also, I forgot to mention, one other easy way to determine if an MC18 is R2/4J or R5/6k is to look at the swingarm.
On the 1989 R5/6k the main extrusion on the swingarm matches that of the frame.
On the 1988 R2/4j they do not match. The swing arm is made from just a plain looking aluminuim RHS.

And yes, as alluded to by Middo and Andy, it is not uncommon to see all sorts of "mash-ups" or "bits'as" after 25-ish years of them being around.
This can make it hard for someone new to the NSR250 to really get a handle on what exactly they are looking at.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...