Datum Line Question

I've noticed in several photos a black Datum line painted on the side of many WWI aircraft. Specifically the Fokkers. Can someone tell me if this black painted line was on all Fokker DVIIIs? Or jus ton some. On some photos and three views they are present and on some they are absent. What is the story with these lines?

Barry in Arkansas

[allanflowers]

I presume this allowed the aircraft to be set up with a level (probably an Abney level) into an accurate position. The adjustment of all the rigging, etc. could then be done with reference to this level position. So it was basically a maintainance reference line. Perhaps the more informed members of this group could weigh in here on this?
Allan