The four "tu56" files here come from four DECtapes read at the LCM+L.
They contain a port of 6th Edition Unix done by Tom Lyon and others
to IBM's System/370. An email from Stephen Jones says:

    Tom Lyon says that they were written using 'tp' on v6 unix pdp-1145.
    No errors on the reads :)

Warren Toomey used the 'tp' utility to extract the contents and create
more modern tar.gz archives of the tape images.

Tom has written a series of articles about this port of Unix on his blog;
the links are:

 + https://akapugs.blog/2018/05/12/370unixpart1/
 + https://akapugs.blog/2018/05/12/370unixpart2/
 + https://akapugs.blog/2018/05/12/370unixpart3/

Local copies of these blog articles are copied here.
