I should also answer the question ...
The rate/rate principle had been identified very early on. It was an ideal that many people working on automatic servo control were heading towards, but was effectively blocked until the advent of modern electronics. From that point of view, it's quite difficult to identify who thought of it first, but the first design is attributed to Andreas Minorsky in Russia. He emigrated to America in 1918, with his designs, and continued work with the U.S. Navy. The first sea trial was in 1923. It was not successful, and development was abandoned.
The German system, developed by Askania, Patin and Siemens, produced the first all-electric system in 1932. It was also the first use of the term, "autopilot". It was very comprehensive in what it did, very "modern", but too slow to respond. Trials were conducted with limited success.
The first successful rate/rate system was based on research by F. W. Meredith at the R.A.E., who commissioned Smiths Instruments to develop it. This became the Mk 9 for the military, and S.E.P. 1 for civil aviation, which was released in 1947.
Guy.