How do I get
inside?
You'll have to resort to trial and error. There is no way to boot the machine
into a single user shell without an external boot device (and this will be
hard to find, as detailed in earlier messages).
I vaguely remember that the RS6000 machines could be booted into a
maintenance mode through use of the keyswitch on the front. It's been a
long while though, and I'm not sure whether all the models had the
keyswitch (the 500 & 900 series certainly did).