I just tried, my VAX 7.2 VMS CD also boots into a
minimal VMS from SYS1
on my MicroVAX. So I guess they changed it between 7.1 and 7.2.
I don't know where they changed but I looked
at a V7.1 binary disk and it had a [SYS1]
directory with many files underneath. The
easy-to-spot difference between S/A BACKUP
and full OpenVMS boot is the number of files.
I don't have either V6.2 or V6.1 to hand so I
don't know if this (undocumented and
unsupported) facility was there back then.
If you tried booting V7.1 and it didn't work,
then either I looked at something other than
V7.1 or you booted incorrectly.
Antonio