There is one producer, about whom we know he produced a lot of crosses, whose mark is unknown and there is one widely spread mark KO, that has no producer.
Btw, KO doesn´t mean Klein & Quenzer AG, Idar-Oberstein. It means Klein, Oberstein. Alfred Quenzer was already dead in 1918, and the Klein&Quenzer AG was founded in 1928. It looks that between 1918 and 1928 Klein used only his name for maker´s mark.
The town Idar-Oberstein didn´t exist until administrative reform in 1933.
PS: I share the opinion that this is correct and original KO cross.