A real toughie here - this odd three-part was used as a replacement for earlier M-3060 Fred M. Locke insulators used on a line in the Sudbury, ON area. It doesn't seem to show up anywhere else that I'm aware of, and I believe only a scattered few examples are floating around in the hobby. I have to assume that a few examples are still lurking in [id=436805812;swamps in the Sudbury area] somewhere.
This example has the "A.F." incuse (partially obscured, unfortunately) on the underside of the uppermost shell, as seen on [id=473724642;this M-2615]. Both pieces likely date to the early-mid 1930s or so. I'm not sure what other marking variations this style occurs in, but I assume it can be found with no markings.
This example came from the collection of the late Alan Hohnhorst and I'm proud to have it. Thanks to Jason Townsend for selling it to me!