The PPK/S is a creature of the Gun Control of 1968. So we start there. It is the marriage of a PPK slide to a PP frame which amassed enough points on a sporting purpose scale for import. If you gun is German made Dr. Rob is correct, at some point Walther adopted the code system where the two digits were replaced by letters a-k. These represent 0-9. My PP is marked IH for 1987.
If your gun has "under license of" in fine print on the slide above the Walther info, it was made in Gadsden, Alabama between 1978 and sometime prior to 2002 by Ranger Mfg. In 2002 S&W took over U.S. production and those guns are so marked.
That's the best I can offer.