Hi Chris,
But we must accept something kept them on! Whilst I agree that Albatros D1--11 and 111 show no OBVIOUS signs (and some superficial signs of fabric covers) Dv photos clearly show them. (page 19 Jasta 5 vol/2 Windsock) and artwork (sometimes)
I am not 100% in disagreement Chris---and like you feel others will know with certainty---but I won't discount the NASM book out of hand---it has to be treat with respect (if only 'cos my copy is falling to bits!
Cheers,
Dave.