Here's another submission from Dave Kurns. It's a common CD 154 Hemingray 42. Take note of the big black-looking blotch. It is NOT junk in the glass. NOT amber swirling. NOT a blob of "snot". NOT a potstone. It's not anything in the glass.

What is on the glass?

What appears black in this photo is actually gray metal that apparently adhered to the insulator at the factory. Dave suspects the metal might be part of the insulator's mold. Is that possible? Has anyone else found an insulator like this?