There is a little bit of info on the Disk III and its use on a III+ here:

http://www.vintagemacworld.com/drives.html

but as for the Disk II, vintage photos show Apple employees with Apple III's with Disk II's hooked right to them (i.e. http://www.digibarn.com/collections/systems/appleIII/sandersinterview.html ). I too seem to recall there's an analog board difference, but the only functional difference is the "disk swapped" sensor to let the OS know the door had been opened.