the N/T/R 21 2000 B is the off road and its 6 port and standard plug
the N/T/R 21 2000 R is a 7 port and i think its either an on road or Rally engine its Turbo plug.
I have a R212000BF they arent easy to find anymore. the 21 R 2000s are really rare and id like to pick one up.
I broke mine in about halfway and the front bearing took a crap and it crankwalked. so i just now put front and rear ceramics from TKO in it. Im going to finish breakin and strap it on an xdyno to see what it makes. I have a extremely modified original N21B 3 port and this thing stock is about the same performance.
The 2000 series is a mystery though, Novarossi acts as if they dont know anything about them.