Harmonium Scale Changer/Coupler Model #3 Mechanism Schematic.

Three Styles of Key Construction 

The Solid Key:  The Key top and damper pads are mounted on a single shaft, and there is a vertical center pin hinge and a single guide pin. This style is cheap and simple with the main drawback being that the bass and treble vents don't open evenly.

The Stick and Pallet:  With this style, the pallet lift is parallel to the sound board, and allows for a balanced air flow. 

The Stick, Lever, and Pallet: This style is  only found on expensive, 3 bank, coupler, scale/changer model harmoniums. There are more moving  parts involved, including the Coupler Sticks which run diagonally underneath the keys, and lift against the shoulder of the corresponding damper an octave below.