The pre-2500 "A" seies are easy to identify from a picture. The music rack was mounted to the underside of the cover lid, and folded lengthwise. The case was narrower so that the pedal switch box was flush with the back legs. They switched to the wider, more familiar "B" style case after the 2500 figure. At that time, serial numbers were more predictable and consistent, and had not taken on the esoteric qualities of later eras.