"U.S. Patent #1,648,402, applied for in the United States on 7 May 1921 and issued on 8 November 1927. In the Netherlands, this was applied for on 14 January 1920 and issued as patent #6976 on 24 February 1922.
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"The Hague Concerts" by Philip R. Coursey, The Wireless World, 30 April 1921, page 65.