My first bike was a '66 YM1. It was made from 1964-66, for model years 1965-67. It seems quaint at this distance in time, but it was quicker than the very popular 305 Superhawk. The YM1 has almost disappeared from the face of the earth, while the Super Hawk, with its much superior finish and build quality, remains very collectible,
I;m sure you mean the 305YM1, if this is your motorcycle then it was made in 1967. Go to this site and check to make sure.
Google.com, type in 1967 yamaha 305ym1
