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Sorry the image isn't better, but here is an interesting "plug cap and base" that is neither parallel nor tandem. The base is porcelain, the cap is "composition", and the manufacturer is identified as "C-H", which must be either Cutler-Hammer or Crouse-Hinds (actually I am pretty sure it is Cutler-Hammer). I think these were called "loop-prong" devices, since the '36 GE Supply catalog shows an adaptor that goes from this to a T-slot, and uses this name.

I remember seeing these in a pre-1910 catalog, and the advantage claimed was that the cord could be yanked from any direction and the plug would pull out without damage. But look at the exposure of those contacts!
Yikes!

-yaktx
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