The Transistortester is a device for identifying and checking electronic components. It also offers additional functionality like a PWM signal generator for example. And it's OSHW.
This repo is meant to provide a convenient way to download firmware packages, documentation files, PCB files and more. Therefore it's structured differently from the main repo.
The project's webpage is https://www.mikrocontroller.net/articles/AVR_Transistortester and the main repo is https://github.com/kubi48/TransistorTester-source.
BTW, all the inexpensive Transistortesters sold on your favourite shopping platform are based on this project and can be updated with the original OSHW firmware versions.