I made a 3D-printable key for adjusting the PO-400 knobs.
The file for the key is available on Shapeways for download, or you can print your own using Shapeways’ materials (the prints are at-cost with no markup): https://www.shapeways.com/product/G7PTELHES/po-400-key-v1-1
I found that the sequencer on the PO-400 was a pain in the ass to use because of how close the knobs are to each other, and how often they bind against the housing. These keys are designed to easily slide over the knobs and provide a larger surface to make more detailed adjustments without disturbing the other knobs.
If anyone has any other requests, I can put together some other designs for tools/components related to the PO-400, Pocket Operators, OP-1, or OP-Z devices.
Video showing the key in use, and a picture of the grey and red printed versions here: https://imgur.com/a/3UPH93B