This is a handmade four channel resistance mixer. Four mono inputs are combined to one mono output. There is a volume control for each input as well as a master volume control.
The circuit is completely passive and requires no power. It is housed in a rugged raw aluminum die cast case.
The design is quite versatile and can also be used in reverse as a 4-way splitter. Also the knobs can be pulled off and reorientated effectively moving the inputs to the back or side depending on what works best for your setup.
Here is an example of this mixer with some other devices…