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Ok, I see that you neophytes are going to force me to wear my fingers to the bone typing!
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This is what Jan Still says in THIRD REICH LUGERS:
"Some Lugers have a Ku suffix or prefix added to the serial number on the left receiver. Essentially all are dated 41 (with exceptions-HC) and almost all bear the Mauser assigned E/655 acceptance stamp on the barrel. Most have at least one part that is marked with the S/42 or 42 Mauser replacement part code.
The meaning of the Ku and the designation of the Lugers on which it is found has long been the subject of much speculation. Randall Gibson, in his book THE KRIEGHOFF PARABELLUM (pp129-136), thoroughly reviews the subject and finally concludes that they were made up of Mauser parts and probably assembled for the Luftwaffe. Lou Cuta (AUTO MAG, Vol. XIV, p.148) discovered that the probable meaning of the Ku was "Kummerer", which means substandard or missized. The proof found on these Lugers has the appearance of other Luftwaffe markings and is likely a Luftwaffe proof.
While it is apparent that the Ku Lugers were assembled for the Luftwaffe from leftover or missized parts, the time and place of assembly has not been satisfactorily established. A 1941 assembly by Mauser is the most likely; however, the finish is cruder than that of the 1941 Mauser Army contract. Some suggest an assembly date as late as 1944. Assembly at a different plant is also possible. According to Albert Speer, Minister of Armaments (in his book INFILTRATION), there were plans in July, 1942, to continue production of the P.08 at the Neuengamme Concentration Camp. It is estimated that about 5,000 Ku Lugers were assembled by Mauser or at another plant sometime in the 1941 to 1944 time span."
I just finished reading chapter 9 of Gibson's book. His conclusion, after exhaustive examination of available material is:
"With firm documentation lacking, it is not possible to firmly state that the Ku weapons were assembled by Krieghoff for the Luftwaffe. It can be stated that Mauser manufactured the components with the facts indicating that Krieghoff was involved in the final assembly and finishing. This being the case, the pistols were most likely intended for distribution to the Luftwaffe."