I have now noticed some differences between a static WW1 Flak gun and the wheeled 8,8 cm K-Flak (or 8 cm version).
I believe that the Flak gun on a static mounting below is a 10,5 cm orfest. An important feature is a tapered diameter on the barrel. On the 8,8 cm K-Flak there is instead a clean diameter drop. The 8,8 cm K-Flak also features lugs (with fastening holes) under the barrel to fasten the gun support during travel while the static (“ortfest”) gun does not have these attachment points.
1) Pictures of the static gun from this thread: http://forum.axishistory.com/viewtopic. ... 0&t=208066

2) 8,8 cm K-Flak (from ebay):

3) A WW1 Flak gun « auf Kreuzlafette » : it features the clean diameter drop and the lugs with attachment points. Therefore, if my assumptions of the differences are correct, the WW1 guns « auf Kreuzlafette » would indeed be 8,8 cm K-Flak Krupp guns.

Emmanuel