Here's the script:

    #!/bin/sh
    i=0
    f=`head -1 $1`
    n=`tail -1 $1`
    rm -f $f
    while [ $i != $n ]; do
        cat $1.$i >> $f
        i=`expr $i + 1`
        if [ $i != $n ]; then printf "5" >> $f; fi
    done

The iteration is done with a while loop based on metatdata from the "compressed" file (the number of parts, $n) and the name of the archive ($1). The order of the parts is determined by incrementing $i up to $n, not from eg, the order of the files in the file system, which seems to be what was implied in the comment earlier.