On the back of the board we discover:

ARM®32-bit Cortex®-M0 CPU

Broadcom (formerly SiByte) 4407 DA9N1S microprocessor

A breakout board for the second shoulder trigger, which is attached separately and is therefore easier to repair.

This one seems to use some sort of rotary knob offering not only an on/off status but a gradual push depth of the shoulder trigger.