The gun control act of 1968 just barely made the PPk unimportable. Walther solved the problem by putting the PPk slide on the PP frame, making it just large enough to pass.
Sometime around 1980 or so, Interarms, the importer of Walther at the time, arranged to have the PPK/s made in the U.S.
They were manufactured by a company in Gadsden, Alabama. There were some initial problems with quality but the were eventually corrected to the point that the Alabama made guns were even being exported to Germany.
After WWII, Walther moved manufacture to Manurhin in France. These guns were if anything even better made than the German ones. Walther eventually began to make the P-38 again in Germany although they kept having the PP and PPK's made in France then shipped to Germany where they were stamped "made in Germany".
OK, that's where I got it. I read an article sometime back and it did mention the slide change. S&W has exclusive import rights today and that's where that came from.
It's good to know that when you screw up, you can figure out why. ;)
Thanks!