never seen a RB Products 3 port With MES 3port exhaust before, if thats old parts, it has never been sold before :-)
Intrigued :-)
A few years ago they did the X10 which was a 3 port:
http://www.rbproducts.com/explose/RB.../01010-X10.pdf
Probably based around this but with a 14.5m crank?