I am Bastiaan Ekeler, this is my work.
Game Boy Camera Canon EF Lens Mount
I designed and built a Canon EF Mount for my Game Boy Camera. The GBC has a sensor size of about 3.6mm² which seems equivalent to a 1/4" sensor (Wikipedia). This gives the GBC a crop factor of about 10.81. With my 70-200 f4 mounted on a 1.4x extender, this gives me a max equivalent focal distance of about 200x1.4x10.81=3,026.8mm.