The seq provided in the GNU toolset has a -w flag to turn on "equal width" mode, so one can also get zero padded numbers out (from GNU seq) by turning on that mode and zero padding the input:
$ seq -w 0001 0003
0001
0002
0003
$Nice, this works automatically like this:
$ seq -w 98 102
098
099
100
101
102 seq -f %04g 0 3
echo {0000..0003} # if bash
So instead of scripting, he could have generated a sorted list of numbers from the files he had. Created a file with the sequence of numbers for the range and diffed/commed the whole thing. Voilà...