Using that connector is a neat idea for sets with the super-skinny plugs of the 50s and 60s.
Most sets that were series-wired AND did not have polarized plugs, had some measures to prevent contact to chassis via metal control shafts, exposed fasthener heads, etc.
Metal-shaft pots were mounted on insulated pieces, nylon fastener inserts hold a hot chassis to metal cabinets without making contact, etc. Fishpaper-cardboard to prevent chassis contact via vent slots...
I hate the older molded, non-polarized extension cords, they never make proper and full contact with plug blades. I have a few that were overheated when space heaters were plugged into them, kept in my show-n-tell box of goodies.
Lots of bad wiring devices were made in the 1960's
The polarized ends are not much better. Then there were the predecessors to these....
http://crnchy.com/wp-content/uploads...er-outlet.jpeg