21 & 28 frames are the same as far as mounting the pipes are concerned, although 21 specific pipes on a 28 will hit the proarm, and 28 specific pipes on a 21 will hit the swingarm on the right.

RS250 pipes won't fit. I had them on our NRS for a while and had to cut off the brackets and weld new ones on.

MC18 pipe mounting positions are different to 21/28, so they may be 18's.