This is a patent that's very interesting for its historical context, and how clear the specification is about the problem it seeks to solve and how it solves it. This was filed in 1929 and issued in 1933 (don't let anyone tell you that long delays in getting a patent are a new thing).
The patent specification succinctly sums up what the problem was in the prior art, "As is well-known, a watch mechanism, or movement, containing parts of magnetic material, or which are sensitive to magnetism, or a magnetic field to which the movement is exposed, may have its accuracy impaired, or indeed under some conditions, its running may be stopped."
Steel was commonly used in the escape wheel to withstand the repeated hits from the pallet fork (pictured above) which may hit the escape wheel teeth three or more times a second. But steel wheels could easily be magnetized, and was susceptible to magnets.
The most common anti-magnetic metal for use in gears was brass, but, "it is not hard enough to resist wear so that while its use for an escape wheel would eliminate the effect of magnetic influences, it would fail for want of wear resisting properties."
So, the inventor came up with a rather elegant solution. "I construct the escape wheel so that it has a main or body part of wheel-form, that is to say, a hub, spokes and rim or circumference, of a non-magnetic material, such as brass, and teeth of steel." There is a small enough amount of steel that it is largely unaffected by magnets, and the brass is just immune to its effects. Further, the combination of the metals, and their different coefficients of expansion as they heat are counteracted by the movement in the plates on which they are mounted.