An attempt at a close-up. I drew a dashed white line along part of the circumferential mold line of the [020] to help identify it. Hopefully these photos are clear enough... it is easy to see the difference in person, but hard to photo.

As you can see, the term "Smooth Base" isn't entirely descriptive (the one on the right is "smooth" in a technical sense as well, isn't it?). But I think the original term "Smooth Base" was used to cover anything that didn't have drip points or corrugations. Later on, other descriptive terms such as CD (Continuous Drip), GB (Grooved Base), and RB (Rounded Base) were introduced to describe their unique features. This left Smooth Base to describe everything that wasn't covered by one of the other terms.