Hmmm ... things start getting messy. Since we have <Set Number>[ - <Optional Qualifier>] - <Individual filename> for the subfiles it might a good idea to change this as well to:
<Set Number>-[<Optional Qualifier>] - <Individual filename>
and consequently use:
<Set Number>-[<Optional Qualifier>] - <Unofficial Part Number>.dat
w.
<Set Number>-[<Optional Qualifier>] - <Individual filename>
and consequently use:
<Set Number>-[<Optional Qualifier>] - <Unofficial Part Number>.dat
w.
LEGO ergo sum