On the 11/45 I recall using the fgront panel to halt
the
processor after the sync sequence.
My notes on an early UNIX were
"System Shutdown
As the operating system keeps some critical data in
core to speed up disk operations, it is necessary
to use the sync(1) command to force this data to be
written to disk before shutting down the system. Due
to interactions with the scheduler it may be necessary
to type sync two or three times to insure that it has
actually been completed before depressing HALT."
--Chuck