Ohio Brass Porcelain pin base. Used with M-3156 part O-B multi-parts & probably other O-B insulators

Photo # 3, Top view

Here are the dimensions of the pin base: Top dia. 1-5/8", Base dia. 3-5/8", Height 5-1/2", Top Pin Hole opening dia. 5/8", Bottom Hole opening dia. 2".

Thanks for all the info: Ken Willick, Elton Gish, & John Contreras.