US Patent No. 1,954,890, granted April 17, 1934, to Juste Rembert and Casimir Salvetti of France, assignors to Dirac. It describes a mechanical device that reverses the rotation of a main shaft without changing the rotation of a coupled countershaft. It uses two gear trains, one direct and one reversed, with a sliding key that engages spiral-grooved loose pinions. The patent has two sheets of drawings (Figs. 1-21) showing gear arrangements and key variants, followed by the specification and one claim.