[finin]

I recall DEC's TOPS10 operating system for the PDP-10 using three character extensions. It was introduced in 1970. I think the older PDP6 and ITS systems at MIT-AI had a different scheme for extensions. On ITS I recall that a file had a name and a version number and you could get the most recent version of the file named FOO by referring to it as "FOO >". Every time you edited FOO, you created a new version whose extension was incremented by one. Periodically you issued a command that deleted all of the old versions.

IIRC I first saw the three-letter extension scheme when I started using TOPS10 in 1974.