The two small carved masks on this mirror sit exactly where the arch begins to turn, and they are there for a structural reason before an ornamental one.
The springing point - where a straight upright becomes a curve - is the hardest junction on any arched frame to make look right. The grain direction changes there, the moulding has to transition from straight to curved, and any slight error in the tangent shows as a kink that the eye picks up immediately.
Placing a carved element at that exact point is the traditional answer, and it is an honest one: the mask covers the junction, gives the curve a visual starting gun, and lets the arch be built as a separate piece properly jointed to the uprights rather than being coaxed out of one length of timber.
Our artisans carve the pair as a handed set, not two copies - one turned very slightly left, the other right - so that they read as looking inward across the arch. It is the difference between two identical faces and two figures attending the same opening.