I think it works like this: you remove the bottom half of the spiral, the clear spiral, and REVERSE it back onto the spool.
Then, the spacing is about right for 16mm/subminiature films.
Of course, there's only one set of grooves remaining on the other spiral, but since submin films are relatively short, that shouldn't be a problem.
Anyway, that's my guess, but I'm not certain.
Perhaps somebody else can answer your question with 100% accuracy.