Trying to find out the year of my tractor

I have a massey fergusson 135 with a 3 cyl gas, the serial number is 9aT80333. I cant seem to find out the exact year, im thinking its late 60s or early 70s, it has 21j3 stamped on the left side of the engine block. Any help is greatly appreciated

Trying to find out the year of my tractor