Calcutta Coupler Mechanism
Calcutta scale changer coupler mechanisms need to allow for the keyboard to shift back and forth and has to sit underneath the keyboard. A series of flat 'sticks' sit underneath the keyboard at an angle with one end under the key and the other end sitting in the lifter mechanism one octave lower than the keyboard end. When engaged, this allows the reeds one octave below the pressed key to sound. The sticks are trapped, barely on some instruments, between nails on the board underneath the keys.
When the coupler handle on the front of the instrument is pressed down, it tilts the front of the keyboard up and the far end down so it can slide back and forth 'underneath' the lifter arms for the reeds.
The. sticks are not very secure and can work their way free and interfere with the key, preventing it from going down far enough to make a sound.
In one case, the harmonium was turned upside down and almost all of the sticks scattered around the keyboard.
The image below shows the coupler sticks underneath the keyboard mechanism. The keyboard is below the picture with the white lifter mechanisms for each key at the bottom of the picture. The lifters for the reeds are at the top of the picture. Note the diagonal scratches in the coupler sticks which helps you put the sticks back in the right place if you had to remove several of them.
The picture to the right shows a close up of the coupler sticks at the low end of the keyboard with the keyboard shifted up four notes.
When a key at the right end of stick is pressed, the associated stick lifts the lifter of the key one octave below the key, allowing this reeds to sound.
This picture shows more details of the coupler mechanism with eight keys and one of the lifters removed to hopefully provide a better view of the mechanism.
The lifters have springs which keep the reed holes covered when the key is not pressed.
This harmonium had four(!) ranks of reeds so the lifter 'shoes' are quite long.
The horizontal row of nails between the 3rd and 4th reed holes keep the lifter 'shoes' aligned and are not the nails which keep the coupler sticks in place.