Here is a close up of the markings of this [id=211126103; spool]. At first, I just saw the incuse PP for Porcelain Products I assume. Then when I got it in the right light, I saw a couple more markings inside two circles touching each other.
The top I might guess is LM in a triangle (Line Materials?) and the bottom WP (??) and both inside the circles.
I can't really tell if this is ink stamped or slightly sand blasted. Because of the gaps in the lines, I'm almost thinking it is sand blasted?
I assume these are all related somehow... Do you have additional information?
ANSWER: Porcelain Products (PP) made the insulator, and Line Materials (LM) sold it. The WP stands for "Wet Process", a method of making porcelain insulators. It was unusual for Porcelain Products to make wet process insulators, so perhaps this is why it is marked.
The PP is incuse marked, and the LM/WP marking is believed to be sand blasted. You can feel a bit of roughness on the surface that would not be present if it was ink stamped.