I doubt that the Ku Lugers were made from rejected parts. More likely they were made from spare parts stocked by higher level armory echelons within the German military. After all, with the switch to the P.38, they would be making parts surplus since replacement pistols would not be Lugers after 1942.
While Mauser likely made most of the parts originally, they would want to be able to distinguish from Mauser made and warranted complete pistols, hence the markings.
The receivers on these pistols range back into the 1930s...
I put together a summary overview of current thought last year. It's posted at:
http://forum.lugerforum.com/showthre...2&highlight=ku
Marc
__________________

Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.