Ok...
Reassured that I wouldn't trash a filesystem accidently, I rebooted it.
And I took better notes this time.  :-)
I was indeed in Stand-alone backup:
41-D  V1.0
F_..E...D...C...B...A...9...8...7...6...5...4...3...2_..1?..
 ?  C  0080  0000.4001
??  1  00C0  0011.700E
 >> BOOT DKA300 
-DKA300
   OpenVMS VAX Version V6.1     Major version id = 1 Minor version id = 0
PLEASE ENTER DATE AND TIME (DD-MMM-YYYY  HH:MM)  25-APR-2001  22:44
Configuring devices . . .
Available device  DKA300:                          device type RZ24
Available device  MKA500:                          device type TZ30
Available device  DKB200:                          device type RZ24
%BACKUP-I-IDENT, Stand-alone BACKUP V6.1; the date is 25-APR-2001 22:45:07:61
$ BACKUP
%CLI-W-INSFPRM, missing command parameters - supply all required parameters
-------------------------------------------------------------------------------
After digesting the above output, I tried booting to DKB200.  That caused
another disk inside to spin up (definitely louder than the first, but not
necesarily to the point of screaming "I'm dead!"), but there's obviously
a problem:
 >> BOOT DKB200 
?42 NOSUCHFILE
?06 HLT INST
    PC = 00000C66
Well...time to research Stand-alone BACKUP, and maybe consider getting a
new disk and some tapes?
I'm booting off of the hard drives.  There's a tape drive in the box, but
I don't have any tapes for it.  There's no CD drive...
Peter