Hi Dave,
I found the below online from Greg VanWyngarden:
Kroll of Jasta 24 was photographed with Fokker D.VII (Alb) 660/18, not 666/18. This photo appears on page 35 of Alex Imrie's "German Air Aces of World War One" (Vintage Warbirds No. 8, Arms & Armour 1987) and on page 81 of Neal O'Connor's Volume 2 on Aviation Awards of Imperial Germany, Prussia.(1990). The serial is partially obscured but it is clearly 660/18.The photo shows no special markings.