Tried googling and didn't turn up much.
Details on page 16 of http://www.classiccmp.org/cini/pdf/Apple/Apple%20II%20Refere...
An exception is the "Reset" key which is hard-wired to the reset pin of the 6502. On early Apple II models, pushing "Reset", which is located just above Return, had disastrous consequences. People used to put a washer under the Reset key to make it harder to push by accident. Magazines published a simple hardware mod that required Control to be held at the same time as Reset, and later versions of the Apple II came with the mod built-in.
The Apple IIe keyboard adds two "apple" keys on both sides of the space bar which are dealt with in a completely different way, and are mapped to the same locations as the two joystick/paddle buttons. This was useful for playing games which required repetitive button-mashing, as it is easier to type a key quickly than to push a joystick button quickly. Since I was used to controlling the joystick with one hand and pushing a key on the keyboard with another hand, I had no problem adapting to early Macintosh software that required shift-click, option-click and command-click to overcome the limitation of a one-button mouse.
"The Macintosh mouse has four buttons, it's just that three of them are on the keyboard."