If one were to flip the picture of the previous insulator's incusing 180 degrees, then it looks like it is a date, I would venture a guess of it being Oct 3 1919 or Oct 5 1919. It appears that the incusing stamp was held at an angle, favoring the bottoms of the characters.

I cropped out the incused marking, enlarged it, and increased the contrast, in order to make it somewhat more legible