Hi RK,
Well, that would roughly translate as 'poor' or 'troublesome'. Not a mark I would like on my Luger <img border="0" title="" alt="[Wink]" src="wink.gif" />
The problem with U/umlaut is indeed that it's difficult to match a German phrase with it, that also makes sense in common German gunsmithing.
That is why I prefer the theory that a small group or company designated as 'Ku' reworked some rejects or leftovers as it fits in line with the 'Kl' lugers that appear to point to a small reworking firm as well.
(I won't even start to discuss the 'kustenfliegerstaffeln'

)
I am looking into another interesting link. One of the few arms companies that fits the 'KU' description is a royal hungarian plant (or Konichlich Ungarnische Eisen- Stahl- und) Maschinenfabrik).
The Hungarians were quite active in 1941-1942 when they allied with the Germans and as a result their airforce was placed under German Luftwaffe command in 1942.
Now for the serious speculation part:
So there suddenly appears a 'new' airforce part that needs to be armed while Krieghoff can hardly supply the German Airforce alone. Imagine that P08 leftovers and rejects from Mauser are shipped to Hungaria where they are overhauled by the Konichlich Ungarian E.S.M. factory, who already were producing ammo on DWM-supplied machinery.
Since these weapons were all Mauser-numbered and coded, some code had to be added to link the overhauled guns to the Hungarian plant, after which they were accepted by the Hungarian airforce (under German control, so with Luftwaffe acceptance stamps).
This may also explain the unknown Mauser deliveries in 1942
Nice theory, eh?