I found this pin with a large, modern "sky yuck" radio-treated porcelain insulator. The pin has a rubber boot covering the threads that looks like it was "shrink-wrapped" into place. [id=370710758] [id=370710882] The insulator is about 8" in diameter and marked PPC2/01401/55-6 at the lower front skirt. A fingernail-size surface glaze flake at the right base edge is the only damage.

Inquiries welcome.