Hello Rod!
Thank you for the photos!
Aviation was a new thing in WW1 and so there was no need to paint the RED CROSS on roofs before this war. I´m sure that hospitals had something like a RED CROSS flag over the building and some signs on the walls. Lorries carried painted crosses on the sides or so.
So when did the different nations start to paint the RED CROSS on roofs to clearify the use of the buildung? Was there something like an international agreement or standarisation?
Or is it really a silly question?