Amazing work.
Guessing core material should not be super critical because of the amount of feedback around it. TBH I have always guessed that Rupert did it this way as a cost-saving measure, an audio transformer wound on a cheap power transformer core with lots of feedback is cheaper and easier to mass-produce than a layered audio transformer.
Can you share some details about the construction? Is it a ferrite, a strip of GOSS (like a power tx), or something higher perm?
