The label reference to 'F.W.G' is Frederick William Green, Honorary Keeper of Antiquities at the Fitzwilliam Museum (1908-1949). Green gave the Museum several Benin pieces and this is likely to be another. It was also labelled as 'Benin' while on display in the drawer under the case between 1989- 2016.
Like the commemorative heads he gave, it may also have been acquired during the 1898 Punitive Expedition