I don’t get it either. I think it’s not really a 370Z-joke; the “Usurper” license plate was originally meant for the Datsun 240Z. Maggie’s other cars have “Thief” and “Shiver” plates, so “Usurper” is probably another reference to one of her books, maybe “The Raven King” (the new one), or maybe to a - yet to be written…