Complex sentence: Some 1960s electronic organs used reverse colors or gray sharps or naturals to indicate the lower part (s) of a split or'broken' keyboard: one which is divided into two parts, each of which produces a different Registration or sound.
Split into simpler sentences: Some 1960s electronic organs used reverse colors or gray sharps or naturals to indicate the lower part (s) of a split keyboard. A split keyboard is a single keyboard which is divided into two parts, each of which produces a different Registration or sound.