“With thanks to Paul Laughton, in collaboration with Dr. Bruce Damer, founder and curator of the Digibarn Computer Museum, and with the permission of Apple Inc., [the Computer History Museum is] pleased to make available the 1978 source code of Apple II DOS for non-commercial use. This material is Copyright © 1978 Apple Inc., and may not be reproduced without permission from Apple.”
http://www.computerhistory.org/atchm/apple-ii-dos-source-code/
Thanks to Ken Gagne and Twitter friends.