United Tire & Rubber Company, Minnesota, USA ???
Last seen: 01-May 1925
Unitone was firmly registered as a trademark of United Tire & Rubber Company.
One possibility is that by May 1925, United Tire commissioned a 'Unitone' branded tube.
However, after careful scrutiny, we see United Tire selling only authorized Radiotron or Cunningham tubes in their advertising.
Perhaps they were warned by the notorious spies that RCA had lurking at many corner radio store? More likely it was they who warned some New Jersey bootleggers.
Drop me a note if you can add anything to this odd brand.