Hello Fred from Florida !
not all islands ("Kampe") in the river "Weichsel" were denoted in the maps, found out that there were many businesses which dealt with clay and pottery in that area, couldn't it be that there was an island in the Graudenz area where clay (=Rot..) was mined?
loaded down many maps of that area, always "Kampe" is the term for a river island.
hope it helps!
greetings from Germany