Here is an unusual insulator. I'm very confident this is an insulator -- It appears to be an insulator and pin in one! The base is exactly the same diameter and length of a typical through crossarm pin (as shown). It has two ridges that would wedge in the hole and a slot that would allow a nail or peg to hold it in place!

Front is marked (Embossed): PYREX / MADE IN / U.S.A.

Back: E-61

Note that this is solid glass -- the flash makes it look hollow.