After a long and interesting search I uncovered a set of 9-track tapes which appear to be a snapshot of the MIT CADR Lisp machine source code from around 1980. This is not the final source code and not the last source release I will make. It is, however, the first source release.
Tom Knight and others at MIT helped me secure permission from MIT's Patent office to release the software. I am indebted to him and the others for making this possible.
On 2016-01-07, it was unreachable. The text above can also be found on http://www.unlambda.com/mit/index.html
, a CADR emulator lives on http://www.unlambda.com/cadr/index.html