On 25th of november, 1910, Andrew W. Proctor of New York, files a strange invention at the Patent Office. Registered July 25, 1911, the patent US998985 is simply entitled « Whistle », but refers to a certain "improvement" to nose flutes.
The base of Proctor's invention is a nose flute quite similar to the first Humanatone, but with a rounded upper lip rest.
But to the flute is added a curious device, a flat metal spring screwed on the nose bulb and ended with a revoluble damper : by pressing on the spring, the operator can mute the sound, and thus, create a tremolo.
« By pressing against the spring 15 so as to move the damper 17 toward and from the opening 11, as above described, the damper is brought partially into the path of travel of air escaping from the passage 10. The damper is thus caused to turn and in turning into different positions it offers different degrees of obstruction to the escape of air from the passage 10. The result is that the whistling sound is modified and gives a rapid and peculiar throbbing. »
I don't know if the invention was released to the public, but it works. Grab a flute, quickly clog and release the mouth hole with your index, and you'll get a perfect vibrato.
The Proctor's novelty was referenced by two later patents : a "warbling musical whistle" (US2996839) in 1961 and an "auto-pipe" (US3745871) in 1973.