It is a geniune mistake (and a bit of a thickheaded one, you'd suspect that Mauser knew what a 9mm looked like...). The photo shows one of the 'mock up' pistols that were actually Swiss 06/29 pistols, dressed up to look like Mauser Parabellums, one being converted to 9mm. Only 3 were reworked for press presentation purposes, one is currently in the hands of a US collector. These 3 were numbered as 10.00.10, 10.00.11 and 10.00.12. The last one was originally Swiss pistol P78220.
Apart from those 'reworked' Swiss pistols, the Mauser R&D departement built a few Swiss style pistols by hand from the drawings Mauser acquired.
Shown here is a remark on the original blueprint that was sent to Sam Cummings of Interarms for approval. He pointed out the mistake in caliber designation, but missed the other one. Ever heard of the caliber '.22 para' ?
