The 'nsys' tape was overwritten at least twice. It was originally a filesystem
backup of a machine used by the patent department to typeset patent documents,
and possibly by other departments to run jobs of some kind. At some point in
1973, it was overwritten with an earlier version of the nsys kernel source
code, and finally overwritten again with the current version of the nsys
kernel.

I have recovered all the recoverable files from the earlier nsys kernel (not
many), and they are in the 'prev_nsys' directory.

I have also attempted to recover all the files from the filesystem backup.
However, due to the fact that the files are all fragmented and none of the
inodes have survived, there is no guarantee of correctness. The original
filenames are long gone, so the files have been given generic names like f01,
f02, and so on. These files can be found in the 'old_fs' directory. All
recovered assembly source files are in the 'asm' subdirectory, recovered shell
scripts are in the 'rc' subdirectory, and patents/papers in *roff format are
in the 'paper' subdirectory. Additionally, I managed to recover some directory
fragments and block maps, which are stored in '_fs_dir' and '_fs_blkmap',
respectively.

The directory format used by this filesystem is somewhat interesting. Each
entry (seemingly) consists of an 8-character filename, a 32-bit timestamp
(??), and a 16-bit file size (??). This does not match either the UNIX V1-V3
directory format or the UNIX V4 directory format. The filesystem backup may
date back to 1971 or 1972, based on the fact that the assembly source files
are for the PDP-11/20.