The options of the tar command is wrong in the restore script. This works:
tar xfv /dev/st0
The only difference is the missing minus, however that is quite important. If you use the '-' then you are using the usual GNU options syntax. In that case the 'f' option must be followed by the file name. In your script 'f' is followed by a single 'v' letter. So tar tried to extract from the file named 'v'. Which does not exist as the error message correctly says.
If the first argument does not start with the '-', than you are using the old, but quite comfortable tar syntax. In this case the required arguments of the options follow the entire set of the options. Now 'v' means verbose, as expected, and the source file is the tape drive (/dev/st0), as expected.
From the GNU tar documentation at http://www.gnu.org/software/tar/manual/tar.html#SEC36:
As far as we know, all tar programs, GNU and non-GNU, support old
options: that is, if the first argument does not start with '-', it is
assumed to specify option letters. GNU tar supports old options not
only for historical reasons, but also because many people are used to
them. If the first argument does not start with a dash, you are
announcing the old option style instead of the short option style; old
options are decoded differently.
Like short options, old options are single letters. However, old
options must be written together as a single clumped set, without
spaces separating them or dashes preceding them. This set of letters
must be the first to appear on the command line, after the tar program
name and some white space; old options cannot appear anywhere else.
The letter of an old option is exactly the same letter as the
corresponding short option. For example, the old option 't' is the
same as the short option '-t', and consequently, the same as the long
option '--list'. So for example, the command 'tar cv' specifies the
option '-v' in addition to the operation '-c'.
When options that need arguments are given together with the command,
all the associated arguments follow, in the same order as the options.
Thus, the example given previously could also be written in the old
style as follows:
$ tar cvbf 20 /dev/rmt0
Here, '20' is the argument of '-b' and '/dev/rmt0' is the argument of
'-f'.