This is a Control Port Switch for the Commodore C64. It provides a rapid fire function. Other than an auto fire function, it fires only as long as the fire button is pressed. And the firing is synchronized with pressing this button. The delay is <= 1msec - the human reaction time is about 300msec.
The micro controller is a (Arduino) Pro Micro. This way, no fancy tools are required for uploading the firmware (just a micro usb B cable).
Find some info on the required cable making on my website
Those are mentioned in the documentation.
The price for the components and PCB was calculated. It is 20.49€ without the 3D printed case (as of June 8th, 2020). The components can be bought cheaper, in case the Pro Micro is bought from AliExpress or Ebay (non domestic).