US Patent 4,114,500
United States Patent and Trademark Office
University of Iowa Research Foundation (Iowa City, IA, US)
A pad for closing the tone hole of a woodwind instrument is formed of a disc of silicone rubber having a compression set at room temperature after 22 hours no greater than about 5%.The edge surfaces of the disc are tapered, and a backing disc of sheet material is provided to facilitate installation of the elastomer disc in many instruments.
Norbeck, Edwin Pad for woodwind musical instrument. US Patent 4,114,500 filed May 20, 1976 and issued September 19, 1978. https://ir.uiowa.edu/patents/12