Here is a close up of the [id=217010767; previous picture]. To the right of it, is the drawing from the patent for in, patented on January 3, 1871 by John Garity of East Birmingham, PA.

http://reference.insulators.info/patents/detail/?patent=U-110645

Pretty amazing... The patent discusses two types, one with a hex head for use with a wrench, and the other with a round head. Two examples of the former are known, and none of the latter.