Mark Evans found a CD 732 with a line on the dome. I have about a half dozen of these with various clarity of the line. I also have many with crud on the dome. This to me is nothing more than a mold flaw. The line does not have serifs or a crisp beginning or end. On some of the ones that I have, it appears almost as a dot in the middle and then barely extends as a line as it fades away. I have seen unique mold marks on many threadless insulators. I believe that is what this is. But, I have no documentation one way or the other.