I can't help withe the date of manufacture, but your Walther PP was made in Mulhouse France by Manurhin for Walther.
After the war, Walther had the PP series pistols made by Manurhin. The guns were shipped over the border to Ulm German where they were proofed, stamped "Made in Germany" and blued.
In Europe the country where the proof test is done is considered to be the country of manufacture.
After the war, Walther didn't have a production plant and weren't allowed to make guns. They had the production plans so they contracted manufacture to the French.
The early guns were marked as made by Manurhin in France "Under License of Walther".
When Walther got a plant up and running, the quality and price of the French guns were so good, they left production with Manurhin.
After that decision, the guns were marked with full Walther marks and as made in Germany with no mention of Manurhin.