Delhi Style Coupler Mechanisms
I am not a fan of couplers, which are mechanical systems which couples one key to the key an octave below the key which is being played. If your harmonium has a coupler, there will be a lever on the far right side past the keyboard. Pushing the lever forward engages the coupler and pulling it back disengages it.
While you get a fuller sound with more reeds playing, the mechanism can create clicks when played. Also, and this is the worst part, if a key is lifted too far for some reason, the tab on the key which allows the rod to pull down the key ends up below the tab instead of above it, preventing the key from being pressed at all.
The pictures below shows the coupler parts underneath the keyboard with the front trim board removed. The bottoms of the keys are at the top of this picture.
Below are two pictures of a Delhi coupler mechanism after removal.
The first shows the full mechanism. The straight part of each rod at the top of the picture rests on a wooden tab on the bottom of the key shown being lifted in place in the picture above. The seventh rod from the top is lifted to show the corresponding curved part of the rod at the bottom of the picture (12 keys lower) is also lifted. This is the non-coupled position, though exaggerated.
When the mechanism is activate and a key is pressed, the straight part of the rod (shown in the picture above on the top-right) is allowed to moved down, which 'rotates' the rod and presses down on the wooden tab of the key one octave above, causing it to sound. The movement of the 'coupled' key is very slight, but enough to overcome the spring force holding the key down at the far end allowing air flow to the reeds.
This picture shows just over an octave of the coupler mechanism with one of the rods lifted.
The picture on the right shows the left end of the coupler arm in the center of the keyboard. A small screw holds that end down. When the coupler lever is back, disabling the coupler, the top part of the "U" lifts up disabling the coupler mechanism.
When removing a coupler, you can remove the screw, and start pulling out the whole. I bend this rod back and forth on the right end of the keyboard (before where the rod goes through the block on the right end of the keyboard and bends up to form the handle) a bunch of times until it breaks. Then I can remove the whole rod.
This video from Old Delhi Music shows how to remove a coupler.