Mission Engineering has released an expression pedal specifically designed to work with the H9. The SP-H9 has a potentiometer wired as a rheostat and an integrated momentary switch connected to the ring. This allows the customer to connect the expression pedal to the H9 with a single TRS cable and get both expression pedal and switch functionality in the one unit. There is also a parallel switch input so customers can connect an external TS switch just using a regular 1/4” instrument cable. http://missionengineering.com/?product=sp-h9-eventide
If I were starting over from scratch, I’d give this one some serious consideration. I wonder was that “extra” hole is on the left side. Prototype shown, or future expansion? Aesthetics? It’s difficult to tell at a glance.