The difference between the 1 speed and variable speed linkage is the plastic cam on the motor arm. The rest of the linkage is the same in a given year. The linkage and pivots are unique on the '63 and also '64 models. '65 and '66 will interchange however, but, with the above note on the cam. I haven't tried to swap the '63 motor with the '64-up motor but the obvious difference I see is the length of the wiring pigtail. The number on the motor is different too. I've swapped the whole '65-6 assembly into a '64 model to get ride of the bolt-on wiper arms, and had no problem doing so but if you're going to try to mix and match, you're on your own!