An optical position transducer is provided for each of the black/white keys incorporated in a composite keyboard musical instrument; the optical position transducer includes a photo-coupler radiating a light beam, an optical filter moved across the light beam, a recess formed in the black/white key at a proper position where the optical filter is aligned with a target trajectory crossing the light beam and an elastic coupler pressed to an inner surface defining the recess and secured to the optical filter so that a worker assembles the optical filter with the key by inserting the elastic coupler into the recess.