The project is based on EnterMice firmware.
Used only a joystick emulation mode.
Miniaturized to the size of the plug.
Suitable to Atari, Commodore, ZX Spectrum Kempston.
Not working with Amstrad/Schneider CPC.
MSX and Sam/Coupe need to use a twister cable.
Timex TC2048 need to change internal Kempston interface (in original has not a ground).
I tried to play with mouse on that
video