As described in this question, Multics used
> as the separator between components in pathnames, and
< as a parent directory indicator in relative paths.
However, an early paper describing the implementation of the Multics filesystem uses
: instead (and provides no indication of how parent directories were referenced). My understanding is that this paper describes the state of the Multics implementation as of "phase 0.5", which was a simulation of components of Multics running under an emulator on a GE635 so that work could begin before the actual GE645 hardware was ready. This work was completed in March 1967, and at some point after that work on the actual hardware begun.
The new syntax had been adopted by the time the system command language was described for the programmer's reference in October 1968.
Is there any available documentation as to when and why this change was made?
(Motivation: it may be interesting to know, in light of this question, how long the '>' convention had been in use at the time Bell Labs abandoned the Multics project)