The little Mozart ballpoint, listed by Montblanc as the Hommage à W. A. Mozart, takes only the short standard refill of type M21. These refills come in F and M, but not in B. The reason is their small capacity: a broad ball lays down so much ink that a refill this short would be written dry in no time.
Anyone after a short pen that will take a fat, large-capacity refill should look at the ballpoints of the Bohème line. They are compact too, but they hold the high-capacity refills and, thanks to their greater diameter, sit noticeably better in the hand. That makes the Bohème usable for longer stretches of writing, whereas the very slim Mozart is really a pen for short notes and for carrying in a jacket pocket.