@ispot the black part is the 3D-printed frame I designed around the HDZ & spindle, so that I can attach things to the front, while still maximising the X travel on my SO3:
(details here)
I am also using the HDZ 80mm spindle mount. The reservoir came with the airbrush, you normally fill it with paint, for short cuts I may use it with a mix of alcohol and water for lubrication. As @BartK mentioned I will probably remove it eventually and plug a dedicated tube to a remote coolant tank instead.