Your description ([nnn,nnn]) reminded me TOPS-10 and indeed, that AMOS link mentions this: "The similarities with DEC's operating systems, particularly TOPS-10, were not lost on Digital Equipment Corporation". So, that file hierarchy model is even earlier, than 70s and 80s. Wikipedia says that TOPS-10 was released in 1967 (back then different name, renamed in 1970). DECsystem-10 was anyway pretty different hardware from modern computer, running from tapes and so on.

Sorry for that offtopic rambling, I am little bit enthusiastic every time, if I see something TOPS10 related.

That's a good point. I don't really remember bothering with directories on any of my 360kb 5.25" floppies in the 80s. When you can only fit 10 files on a disc a hierarchy is usually unwarranted. You hierarchy is those little plastic dividers in your floppy disc storage box.