TIL the Treaty of Versailles ending World War I also included a clause establishing concert pitch as A = 435 Hz

From pages 170 & 171:

[T]he multilateral treaties, conventions and agreements of an economic or technical character enumerated below … shall alone be applied as between Germany and those of the Allied and Associated Powers party thereto:

. . . .

(22) Convention of November 16 and 19, 1885, regarding the establishment of a concert pitch.