This is enlarged from a part of the diagrams from US patent 596652 granted Jan 3, 1898 to Henry M. Brookfield and Seraphin. Kribs. The patent was a press "for making insulators for telegraph lines and is specifically adapted for use in the manufacture of the particular form of insulator ... in which there is a central opening...passing entirely through... the lower end ... is made with a central depression."

Color added to make the diagram easier to understand.