83 A model has no ADR compliance plate, slightly smaller fuel tank (the LHS lower edge is straight across, rather than dropped down), no battery bracket on the frame. The battery bracket on the B models means that the shock reservoir got moved forward, so it's in the way of the carby.
The gearchange mechanism is also different, but you won't see that until you get inside the motor.
The 83 model also has the small shaft for the front sprocket: uses YZ125 sprockets, not the YZ250/KX250 sprockets that all the later KDX200s use.