Nat's birth name was known by Bart *and* Kitty, it was too dangerous for him to stay alive as John Mandrake. He had to die. It's kind of beautiful now that I think about it, how death allowed the memory of the person Nathaniel to endure. Think of how Nat was the only magician who revealed his real name to Bart, the ones before him were remembered by Bart as Solomon or whatever. I think though, that Nat's gravestone will have Mandrake's name on it, unless Kitty intercedes (unlikely imo)
Whoo rambled a bit there.
EDIT: Ohh yeah, Bartimaeus knew Ptolemy as well, but that's less significant to me, because some other pharoahs were called that and it seems more like a family name that way.
Nothing to see here, move along.