When destined for Observatory Timing Contests, Patek Philippe watches - as did those of other makers-- had their movements double-engraved with serial numbers. The inscription "Adjusted Extra" on the bridge of the movement refers to the high finishing of all the parts of the movement. The Guillaume Balance. Anibal acier au nickel pour balanciers, an alloy invented by Dr. Charles Edouard Guillaume, exhibits unusual properties, both in terms of thermal expansion and in changes in elasticity. Around 1900 Guillaume attempted to eliminate the so-called Middle Temperature Error caused by the fact that the change of rate in a timekeeper with a steel-brass bimetallic balance is approximately a linear function of temperature, while the change of rate caused by change in elasticity of a balance spring is approximately a quadratic function. Thus, it equals zero at only two temperatures, causing secondary error. In 1899, Guillaume noted that steel with an addition of 44.4% nickel had a negative square coefficient of thermal expansion. Anibal, combined with brass in bimetallic laminae, makes expansion close to quadratic. Balances with bimetallic rims made of anibal and brass are usually called Guillaume balances. When combined with special balance springs as in the present watch, they exhibit remarkable temperature stability, on occasion not exceeding 1/50 second per day at 1oC.