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).