Great stuff Bob!
That double "L" slot receptacle is curious. Yet another old variation to add to the collection. Presumably it was intended for parallel or tandem blade plugs the same as the double T-slot rather than for a special L-shaped prong plug.
A couple of those recepts (e.g. Pic #10/Fig. 28) also appear to be polarized types. I hadn't realized these were in use in the U.S. as early as this.
