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1938
See Dieter Marschall's book Walther P1-99
You will have to contact Walther for the exact date.
Walther arms Ulm/Donau, Germany
The PPK/s is a PP with a PPK slide.
1931 to date
The Walther PPK pistol is manufactured by German small arms maker Carl Walther GmbH.
No such sn used by Walther.
There were two companies known as Interarms. The first was an importer/exporter, formed in 1953. They did not manufacture firearms. The second - and current - Interarms is a subsidiary of Hi Standard, and does not manufacture a PPK or PPK/S pistol. If what you have is actually a Walther PPK/S, value could range anywhere from $300 - $800, depending on specifics.
Walther uses a date code of two letters.
The Walther PPK pistol was first released in 1931. During that time the Walther PPK pistol was popular amongst European police and civilians because it was reliable and concealable.
You will need to contact Walther