Hi Joe
I believe Sprockets is correct and these items were indeed produced by Electronic & Musical Industries Ltd, England. I have no definite references for this, but I do know that EMI had defence interests as well as their better known musical department. Bonnex will probably be able to give us some more information.
I have two slightly different variants dated 1971, but I have never seen any published information or Data Sheets regarding this Switch, which I presume was a proposed replacement for the Switch No 4 Pull.
Cheers
Switch