To clear any doubts on its legitimacy and whether it was simply a typo on the certificate, I'd advise you to contact Patek directly. I'm reluctant to give out internal numbers for obvious reasons, but we have a boutique ('Maison') right there in Shanghai if you need assistance to get in touch with Patek China.
Hope this helps, and congrats in advance on an exquisite modern reference should you decide to proceed with the purchase!
FabR