The Final Cartridge III was a popular cartridge for the Commodore 64, developed in 1987 by Dutch(?) company Riska B.V. Alongside with disk and tape turbo features, and freezer functionality it offered simple GUI environment. The company encouraged users to write their own programs employing GUI features, they even anounced contest for FCIII programs and sent sample programs and documentation upon request. Unfortunatelly, the idea didn't quite catch on, and programs written specially for Final III are very rare. I was unable to find single such program on the internet.
Back in 90s, when I still had the real hardware working, I started exploring the cartridge internals using built in monitor, trying to write something on my own. I still have my paper notes somewhere. Later, when I got myself a PC, I continued slowly when I had some spare time. Well... the most important part is almost finished, results below.