To the best of my memory, 501-503 were titled "Mathematics" and 504 "Further Mathematics", but it's possible 504 was "Higher Mathematics".
Someone posted a 504 paper here a couple of years ago, and I recall that, for example, there was a question that asked you to prove the Mobius inversion formula. That was something not covered in the Cambridge Maths degree until the 3rd year, (although, to be honest, there's no real reason it couldn't be taught in the first year).