Paul, Considering that this is for 110VAC, would this have been used on construction sites, with a step-down transformer, for powering tools?. The face of this plug reminds me of the old Reyrolle industrial plugs used here that used the "Scraping-Earth" method of effecting an earth connection between the plug and socket.
#123919 - 06/12/0611:14 AMRe: Hubbell, British style
This plug is to the old BS196 standard for single-phase industrial connectors, and one of the big manufacturers in the past was Reyrolle Belmos!
The 110V types were certainly used for portable power tools. Note the keyway on the inside of the shell to prevent cross-connection with other systems. The BS4343 connectors used today didn't start appearing until sometime in the late 1960s as far as I know.