Found this original label pasted on the bellows inside the lid:
"Directions for Using the Base Damper, When the knob at the left of the keyboard is drawn out, the left foot pedal swells the whole instrument. Press in the knob and the pedal swells only the treble, or upper half of the instrument. When the swell pedal is pressed down the knob may be drawn out or pressed in at pleasure to make the base loud or soft."
Found this hand written note made when we received the melodeon in 1989:
"Right knob when pulled out restricts treble keys- making it softer. Left knob (see instructions on bellows) pulled out allows bass to sound louder. Left foot pedal swells the treble & bass if left knob pulled and bass....[remaining not legible]