With thanks to a collector friend for these pics.
Based on the Girandoni/Girardoni ball repeater action, with a tube running down the right side.
The rear knob on the left side of the lock plate moves a pin backwards into the left-most of the three holes in the butt reservoir, locking it and stopping it from unscrewing. The front knob operates an air bypass.
With thanks to John M for pics of the same pistol:
This example is from the Keith Neal collection: