Apple Extended Keyboard II is an old mechanical keyboard with ADB interface. Many people consider it to be the best keyboard ever, even better than IBM Model M. I think it's not really important actually. I've got both of them and can't say which one I like the most.
What is really important, AEK has a few interesting things from engineering as well as design point of view. This project is not about design nor the engineering however. This is an instruction how to assembly/disassembly the keyboard which one can use or just watch the photos and find out the true meaning of engineering and product design.