Hi Clint
With regard to the training units, I think it is no use trying to identify when and where by looking at paint schemes, crosses and such things.
I doubt very much these units gave a toss about front line regulations.
I have talked to Kees, who is also very impressed by this album, even if he feels EUR 2000 is way too much, but then again who doesn't?
He agrees with you about the Halberstadts, and advise most of the C.xx/18 are Rumpler C.I. One exception, the C.20/18 is a DFW C.V
He is convinced it is a training unit, possibly two different ones, but has no theory with regard to the odd "Bestellnummern" visible.
I am beginning to harbour a thought that some aircraft were simply ordered outside the military Bestellnummer system, and in this present case were given a "phony" number just to resemble the mainstream one.
Cheers
Stig