they are the same as the ct1 -3 and the dt1-3 except that the ct ran a 175cc engine and the dt ran a 250cc they only came out for 3 years the at1 at2 and at3 71-73.
the a,c,dt siries bikes were the first of yamaha's "torque induction" engines meaning that they had expansion chambers instead of the previous trumpet exhausts,
and hats about all i know,
i think after they finished being produced they moved on to the it and tt bikes