It's a PPK I bought new in March of 2001. On the left of the slide it says Walther, made in USA, under license of Karl Walther Waffenfabrik; on the right side Walther USA LLC, Springfield, MA. The manual doesn't have the word Interarms anywhere either. Nor S&W.
With the slide off I could see that part 013 is pushed down by the safety lever and releases the hammer to fall, but I absolutely could not press hard enough, even with the blade of a screwdriver, to drop the hammer. If I pull the hammer back a little bit from its natural cocked position, then part o13 will pivot down and the hammer will drop. So the problem is probably in the hammer area, not the safety/de-cocking lever.
There are plenty of possibilities: in the cocked position the hammer is not as far back as it should be; there is an improperly machined part or parts that prevent correct lever action, i.e., the lever can't get past center.
This pistol shoots fine and with maybe 2000 rounds through it has never once failed to fire and cycle. Thanks for the reply. Any further ideas (call a real gunsmith!) much appreciated. I know there are a lot of folks who just like to shoot and some who like to shoot and know how it all functions.
VicM