I recovered my data.
Learn to use 'vim', at least for rudimentary use. It is the code
editor on live CDs because its minimal version is very small. The
enhanced version adds a macro language with a huge library.
Thanks for all your help.
The problem was in fact knoppix, but not because it had an
older version of lvm2 or even lvm1.
The problem was that the human-readable part I extracted from
the beginning of the device contained invisible characters in
front of the
'vol0' label in the file 'volt.txt' .
These characters were not visible in terminal using 'cat',
'more', and they also did not appear in standard error as a
consequence of the 'vgcfgrestore' command, as witnessed by the
first line of the error: