I just spoke with Mike Williams bout this last night at reheasal.
Mike said A double G would be above double C in ANY band he has played in.
He also says that he just calls it a G above double C. But the first double is C.
Mike has been the lead trumpet and scream player in the Count Basie Orchestra for 20 years.
If ANYONE would know, it is he.
Mike comes out front and hits a double C in this tune. (Discomotion)