Complex sentence: It is frequently but incorrectly believed that the Wissower Klinken were a model for the painting; these did not yet exist at the time of the painting's creation, but appeared later because of erosion.
Split into simpler sentences: It is frequently incorrectly believed that the Wissower Klinken were a model for the painting. These did not yet exist at the time of the painting's creation, but appeared later because of erosion.