It is a presrt potentiometer or"trim-pot" used to adjust or calibrate the equipment. These usually feature a screwdriver adjustment rather than a knob.

[![enter image description here][1]][1]

*Figure 1. A selection of preset potentiometers. [Source](https://www.robotroom.com/Parts/Trimpots-quarter-inch.jpg).*



  [1]: https://i.sstatic.net/XRZGw.jpg