In article by Warren Toomey:
> The answer is: UNIX had / and /usr split by at least the time of the
> July 1974 CACM paper ``The UNIX Time-sharing system''.

I just found some more evidence. The 2nd Edition UNIX manual is dated
June 1972, but the actual man pages have their date of last modification.

The manual for init(7), dated 15th June 1972, says:

[ If console switches are set to 173030, a shell is attached to the
  console immediately, i.e single-user mode ]

	Otherwise, init does some housekeeping: the mode of each DECtape
	file is changed to [read-write] (in case the system crashed during
	a tap command); directory /usr is mounted on the RK0 disk; directory
	/sys is mounted on the RK1 disk.

Cheers,
	Warren