The Yamaha SRX-6 came in several incarnations..
In Japan there are 5 types of SRX6 :
3 twinshock & 2 monoshock (and 7 types of SRX4)
First there was the ´85-´86 no nonsence kickstart only version (this is the one most export versions is based on)
Secondly the there is an updated '87 version (Jap market only) with 17" wheels and a single front disc. F-fork, R-shock, carburetor setting, passenger peg, engine updated etc.
3rd is the '88-'89 model wich differs from the '87 one accordingly: chain (from 104 to 132), sprockets (Front:15 to 19, Rear: 37 to 47), carburetor setting, silencer.
4th is the '90 "evo" version, the SRX600E if you will with electric start and redesigned chassis (monoshock) and bodywork.
Finally there is the ´91- version, just a few smaller updates of the ´90 model. This one is still in production i believe.