[SloopJon]

I don't know about DEC-20, but on VMS, changing a file makes a copy with a new version number:

  $ edit foo.txt
  ...
  Ctrl-Z
  $ dir
  foo.txt;1
  $ edit foo.txt
  ...
  Ctrl-Z
  $ dir
  foo.txt;1 foo.txt;2

When you delete a file, you have to specify a version field (blank for latest):

  $ del foo.txt;
  $ dir
  foo.txt;1

IIRC, you can configure how many versions to keep.

[tragomaskhalos]

Yes that's right, the versions to retain was configurable, and the whole versioning mechanism was really useful and a great miss from today's OSes. The VMS file and directory syntax otoh was a real pain, Unix definitely wins there.

[vincent-manis]

The version number was used the same way in TOPS-20 (which I never used) and Tenex, its predecessor (which I did). Emacs still has a facility for numbering backup files in the same way.