As you may know, there is very limited firearm registration in the United States. Firearms sales are recorded / records kept by manufacturers and distributors identifying the retailer to whom it was sold. The retailer keeps records of the person who purchased it from them. Persons who regularly buy and sell and who have a Federal Firearms License keep records. However, there is no known "central data base" which has records of each and every transaction occurring regarding a specific firearm. Prior to 1968, few, if any sales records were kept. The only database known to exist is one that lists stolen firearms, which lists make, model, serial number (if known), and the transaction/transfer history, if known. There are law enforcement only databases , such as "Drugfire" which list the make and model based on forensic exam of recovered bullets, and empty shells. They are often cross matched against the stolen firearms data. There are some local & state jurisdictions that require firearm registration and submission of test bullets & shell casings. However, none of these are available to the general public, or even law enforcement just out of curiosity. INTERPOL and EUROPOL also have firearms databases, some of which do include transfer and ownership data related to the Firearms laws of a specific country or countries. But they are not accessible by, or available to the public. The Walther plant in Germany MAY be able to look up the serial number in its records and identify when your father's PPK was made, and information as to the caliber, and possibly other distinguishing features, such as finish, e.g. blue, nickel, alloy. Perhaps to whom it was shipped to when it left the factory. That would be the limit of available information. Walther may charge a research fee. I wish you luck, but it's not likely you will find the firearm.