Exactly right. The 3-speed was a running change sometime in the late winter/early spring of 1965. Prob'ly done to save money on cheaper motors.Motor is different. The 2 speed is actually a 2 speed motor (3 wires). The 3 speed set up is actually a single speed motor with a three speed switch and a resistor bank in the heater housing to control the speed (2 wires out of the motor).
'66's came with the three speed. I thought all '65's were 2 speed, although some have said late '65's came with three speed. Running change may have varied from plant to plant also.