The implement that Arcimboldo uses to construct mustache and beard of his librarian is , in my opinion, definitely a duster . The clue is in the Oslo version where the handle is quite visible. But, it's not a feather duster, rather this duster is made from pieces of fur from the parts of small animals usually from the tail or the paws of rodents (hare, rabbit, weasel), even foxes. Arcimboldo knew about the dangers of dust first- hand since he was not only court painter of three Habsburg emperors but also art collections manager for two of them. We can be sure that he had recognized dust as an enemy of art collections. I don't know weather his job included the oversight over book collections but we can assume that he knew colleagues who were. In the sixteenth century books were often shelved or stacked with the foredges facing out from the shelves thus making them great dust catchers. So, frequent dusting became a necessity.