> [...] the maker was almost certainly a transcriber who used it to keep his place on the page and note the column he was writing in when he stopped. The wheel would be moved to the stopping point and the circle turned to the number of the column he had been writing in when he stopped.

It would make a lot more sense that the bookmark was placed in the source book rather than in the copy. I.e. the wheel would be turned to the source column they had been reading from.