Zanzibar Scott 17b Gibbons 13l Used Stamp

17b (13l) 1895-96 6a pale brown Q Victoria stamp of India overprinted "Zanzibar" in black, ERROR - "Zanzibarr" (pos 37), massively undercatalogued being one of four known examples, the last used example sold for $17,500, light "Zanzibar" cancel.