Probably a piano score for Glenn Gould, to indicate what he should be humming if he can't contain himself. Basically, it is the line that the piano play is supposed to insinuate (note that it's an octave or two below the lower notes of the sixteenth phrases). If there is any melody instrument playing along with the piano, this would be its track.
And if this is played by a chromatic button accordion, it actually fits in the right hand (assuming that you interpret the 8va in the upper staff to hold for the small notes as well).
My main guess, however, would be that the transcriber proposes how an additional melody instrument could pick a lead to play, even if the original rendition is piano only.