Here is a CD 162 LYNCHBURG 36 signal insulator.

Notice the "warts" protruding from the threads. Not like they were "bubbles" These are open back "warts"

Never seen this phenomenon on any other insulator before.

Something ( a chemical or liquid ? ) was on the thread mandrel as it formed the threads. These "warts" formed but did not seal up in the back side to form bubbles.

Something about the chemistry of the glass. ?

Glass was too viscous (had cooled too much before threads were inserted) ?

Definitely not your ordinary "bubbles" we see all the time.

Thoughts ????