Regarding cropping the tops. I can imagine that the practice to crop the margins was in order to fit the parchment folio size to the available cover size. And, if the two sizes do not differ too much from each other, it is sufficient to crop just one margin to solve the task. (However, I do not know why the choice would always fall onto the top margin).
But I have an impression that the VMS has, at least, its bottom margin cropped. Look e.g. at You are not allowed to view links. Register or Login to view. (and this is not the only botanical folio suggesting that).
But I have an impression that the VMS has, at least, its bottom margin cropped. Look e.g. at You are not allowed to view links. Register or Login to view. (and this is not the only botanical folio suggesting that).