I just came across on the Wikipedia page that this bust is being suggested as the true image of Cassius Longinus, rather than "Corbulo" as previously identified. The reasoning for this is contained in the book by Sheldon Nodelman: "Ancient Portraits In the J Paul Getty Museum" which can be downloaded here.
Much of the reasoning for assigning this to Cassius is contained in footnote 59 on page 59:
I note that the book contains lots of interesting material, not the least of which is this representation of the very modern-looking bust of Cassius' collaborator, Brutus:
