I now have two of these lenses, and both of them have what appears to be a continuous circular separation around the edge of the rear element. Is this a common flaw with these lenses? I have only seen these two, and it seems odd that they would both have the same flaw. I cannot believe they were made this way.

It can be caused by a number of things. Moisture has probably gotten into the mount and affected the balsam cement. One of the biggest causes of this is careless cleaning. Putting the fluid on the lens before wiping. The fluid should go on the cloth, not on the lens. It probably won't affect performance if it's just around the edges.

I have a 15-in. "Tele-Optar," and have unscrewed the rear group to examine it carefully. I don't see the effect you describe, so I don't think the lenses were made that way.

I have one of these lenses, when viewed a little off axis from the rear, the actual ground edge of the rear element can be seen. The ground finish is such that it could look a little like separation. These surfaces are often painted black to reduce their visibility (cosmetic appearance) or to reduce scattered light in other lens designs.