My Dad gave me this insulator around 1969. He worked for the City of Detroit, MI Public Lighting Commission. The streetlights were series back then, and they used (he swears) Hemi 15's and Hemi 45's on the pole tops. His partner was given the task in earlier years to research insulators to be used, and Hemingray said "You're using WHAT". There is ghost embossing all over the dome.