Based on my complete lack of knowledge in this area, I say BS.
Try reading these:
The 'Manurhin myth' explained - WaltherForums
PPK/s DOB - WaltherForums
It sounds to me like no Manurhin made guns bore the Walther banner. Ever.
And this:
http://smith-wessonforum.com/lounge/171770-walther-ppk-need-some-schooling-3.html
After the war Germans were not allowed to manufacture arms. Walther therefore licensed the French company Manufacture de Machines du Haut-Rhin, shortened to Manurhin, to produce PP and PPK pistols. These were produced in the factory at Mulhouse-Bourtzwiller, France not very far from the Walther factory in Ulm, Germany. These guns were clearly identified as being made in France under licence. They did not show the Walther banner.
Later, German companies were allowed to re-commence production of arms. For some time after this Walther made the forgings which were sent to Manurhin for machining. The soft or unhardened slides were then returned to the Walther factory at Ulm where they were roll marked and hardened. They then completed the final assembly, testing and proofing. Under German law, because the final work was completed in W Germany, they were entitled to mark the guns as having been made by Walther in Germany.
Possibly at the same time, Manurhin continued making pistols entirely in France, and these were tested and proofed in France. They will have the St Etienne proof mark. French made guns were not entitled to show the Walther banner.
Tabs will come along soon and correct me.