Looks like it says 69 (for made/proofed in 1969) on the barrel, visible in the ejection port. Also the Ulm/Germany antler mark. I think I can also spy an [NdS] stamp, official use, state of Lower Saxony, i.e. a police gun. I'm unable to make out the Interarms mark you mention, thanks to the quality of the pictures 
But ever since 1968, guns imported into the US need to be marked by the importer, so I'm going to say that's what it is.
So what you got is a Walther PP, made in Germany in 1969, from French supplied parts, issued to the police of the German state of Lower Saxony (Niedersachsen, hence NDS), and imported to the US at some point.
But ever since 1968, guns imported into the US need to be marked by the importer, so I'm going to say that's what it is.So what you got is a Walther PP, made in Germany in 1969, from French supplied parts, issued to the police of the German state of Lower Saxony (Niedersachsen, hence NDS), and imported to the US at some point.
