Here is the CD 731 on the left, CD 731.2 in the middle, and a CD 728 on the right. I've seen a few 728's and 731's being offered on eBay as 731.2's. Also, and certainly nothing against the McDouglad's considering all they've done for this hobby, but the CD 731.2 they picture on page 285 of their A History of North American Glass Pintype Insulators (1990) appears to be a CD 728. They do comment on the distinction in that photo's notes. Looking like a 728 could be due to how the photo was cropped, but there is some confusion about the distinction and I hope this photo I've taken helps in some small way.