OS X :: Split Files With Letter Extensions

I have a set of split files with letter extensions. They are named <filename>.iso.__a, <filename>.iso.__b, and so on. Additionally they continue on to <filename>.iso._aa to <filename>.iso._as. This further confuses me because a Mac will list the <filename>.iso.__a first but a PC will put the <filename>.iso._aa first so I am unsure which is the initial file. I haven't found software that will join them properly. Split and Concat stalls out at 4kb. MacHacha makes a full sized file but it won't mount. Stuffitexpander won't even try, and Ajoiner shades out all the files as they don't end in numbers. I also tried renaming all the files with numbers but this too failed.