US patent 1446758, Thaddeus McHugh, "Neck for musical instruments", issued 1923-02-27 Patent on truss rod. Note that the diagram in McHugh's patent document shows an incorrect design for the truss rod channel. The channel is shallower in the middle, so the truss rod is installed with a backbow. Tightening the rod increases neck relief, acting in concert with string tension, rather than reduce relief in opposition to string tension.
US patent 964660, George D. Laurian, "Stringed musical instrument", issued 1910-07-19 A patent on instrument that is "designed like harp guitar", but it mentions "truss rod" concept "to prevent a head springing under the strain of strings".
