Probably because drawings from the Fokker factory were not available, and the Albatros factories had to work with a pattern aircraft, differences existed between the D.VII’s build by Fokker, Albatros and Ostdeutsche Albatros Werke. From which factory a D.VII originates, can be determined by the serial, or by the type of engine cowling.
Basically, there are four different cowling layouts of D.VII’s. First, there’s the original V.11 and early Fokker built production aircraft, with the twin exhaust pipes on the right side of the 160 hp Mercedes engine. This type of cowling can also be seen on D.VII(Alb.) 527/18, the first Albatros production aircraft. Later Fokker D.VII(F) aircraft had a different exhaust pipe for the 175 hp Mercedes or 185 hp BMW engine, and the cowling had more cooling louvres.
D.VII’s built by Albatros had also more cooling louvres, in a different layout.
The O.A.W. built D.VII’s can be easily identified, since they have cowlings with 20 round shaped cooling openings on both sides. Some sources state that only O.A.W. painted the cowlings in a lozenge camouflage pattern.