The incidental finding seen on the frontal chest x-ray is mitral annular calcification, seen as a C shaped calcification in the lower left parasternal area.

Mitral annular calcification is an atherosclerotic process that involves the mitral annulus in about 10% of the elderly population. It can appear as a circular, ovoid, C or J shaped calcification in the mitral annulus and can lead to mitral incompetence.

Reference: Fundamentals of Diagnostic Radiology by Brant and Helms.