The dominant temperature error in a watch is not expansion of the balance but loss of elasticity in the balance spring as it warms. Left uncorrected, a watch loses substantially over a normal range of temperatures.
The bimetallic compensation balance answers this mechanically. A rim of brass fused to steel, cut near the arms, curves inward as temperature rises, reducing the balance's moment of inertia and speeding it enough to offset the weakened spring.
Adjustment was empirical and skilled. Timing screws in the rim were moved and weighted so that the watch kept the same rate in an oven and in an ice box, and an 'adjusted to temperature' marking records that this work was done.
Materials later made the mechanism unnecessary. Low-expansion and low-thermoelastic alloys developed around the turn of the twentieth century allowed a plain monometallic balance with a compensating spring to outperform bimetallic work while being far cheaper to produce.
For identification this is a dating tool. A cut bimetallic balance with timing screws suggests nineteenth- or early twentieth-century precision practice; a plain smooth balance with a free-sprung or overcoil spring suggests later alloy technology.