OK Gameswat, so it seems more likely it's British.
Unfortunately Nic doesn't state where his image of the Electric Volta was from, so we can only guess that the original source included the D. Harper & Co. attribution. Paul Braithwaite cites this page of Nic's book as his source. I assume Paul had some reason to believe Harper was probably only a distributor as Nic did not say so. It is notable that the label on the ebay example said Supplied by Harper's (not Manufactured). Perhaps this, combined with the fact that we have two quite different shockers from Harper, lends weight to the idea that they were supplying machines from different manufacturers. The only manufacturing mentioned by Grace's Guide is small stuff like "Leather Goods, Albums, Calendars, Blotters, Diaries, Stationery Goods and Advertising Gifts". On the other hand, the Harper Piano Co. were apparently making Singing Birds and Harmonic Synchronising Pianos for Cinema Theatres, amongst other things in the 1920s.