After passage of the 1968 Fun control act...er. Gun Control act..imported firearms had to have the importer's name & address on them. (not the full street address, just city & state or country)
The Walther PPK was banned from import by that same law. Because of the law's arbitrary point system based on combining the gun's measurements, the PPK scored too low (from what I heard it was a 1/4 inch too small)
Walther then created the PPK/S which met the GCA's restrictions, and was allowed to be imported.
IF you have a PPK without import marks, its very likely it entered the US before 1968.