Learning the Art of Electronics - keypad
There's a schematic of the keypad in the old Student Manual at the end of Chapter 15. Not sure what the disembodied U4c bottom-centre means, but it can't  be that hard (famous last words!)

Update on 20 June 2018:

The author, Thomas Hayes, has been away on vacation but has got in touch to say he will put up-to-date details of the keypad on the Learning the Art of Electronics website. It is different from the one in the old Student Manual as he has re-designed it with a microcontroller in place of the many IC's of the older version, to do keypad scanning, switch de-bouncing and display.