Patent dates are not a reliable indicator of development dates. Some companies hold off patenting a product as long as possible to avoid giving the competition too much information too soon. Others like to jump in and patent as many features in as many different ways and as early as possible to shut out competition (Colt and Browning).
That whole era is confusing, with Mauser ownership changing, new developments, a ban on production of 9mm P pistols, etc. It seem evident that Mauser was not really that interested in those guns, though no one today knows why. And there is less known about the numbering and (very) limited production.
Perhaps Mauser held off patenting a new 9mmP pistol for the simple reason that it was illegal to make such a thing in post-WWI Germany.