The iron broken vane part is a quest item used in One Small Favour. It's obtained by climbing onto the roof of Phantuwti Fanstuwi Farsight's house in Seers' Village and examining the weather vane there. The directionals need to be repaired with an iron bar, turning it into directionals. If you lose the broken vane part, Phantuwti will charge you 330 coins. Once repaired, the parts can be returned to their position on top of the house in order to continue with the quest.