Here you see a 1530 (bottom) and a 1530a (top). Note the difference in the kind of finish. Some 1530 drills have a red varnish, but it is still clear, not opaque like paint. Also observe that the 1530a is a bit longer than the 1530, and the chuck is longer.

The details show the primary design difference between the 1530 and the 1530a, where the handle meets the casting. The handle has a ledge that rests on a raised piece of the casting to prevent the handle from twisting in use. On the 1530, the raised portion of the casting is made by cutting through the body, which leaves a weakened spot. The 1530a had a different manufacturing process that made it stronger at that point.

The typical width of the pinion at the red line is 3/8". Some 1530 drills have a beefier pinion.