This is an attempt to show the difference between the CD 251 NEGM [017] and [020] listings. The one on the left is [020], and is indicated as having {MLOB} and SB (smooth base). The one on the right is [017], and is indicated as having {MLOB} and RB (rounded base). The terms "smooth" and "rounded" are a little less than ideally descriptive, but I am not sure how to better describe them really. The idea being conveyed is that an RB has a smooth transition from the skirt to the base, with no sharp transition or mold line to break the smooth transition. Since the mold piece that forms the skirt also forms the base, all RB insulators will also have {MLOB} (mold line over base).

The insulator on the left has a mold line running circumferentially around the base, pointed to by an arrow. This causes a slight break in the curvature of the transition from the skirt to the base. This is the essential feature that defines a "SB" (smooth base). In this particular instance, it also has an {MLOB} as will be apparent in subsequent photos.

Incidentally, for some reason every CD 251 NEGM [017] example I have seen has (uniquely) large embossing on the front as well as the back, so this feature may be used to help identify a CD 251 [017] variant as well.