I just subscribed to Sweden Rock Magazine's digital edition, which includes a fair share of archived issues. They interviewed Bruce in 2018, after his book was released. Apparently they always record their rehearsals with a simple, portable recorder. Bruce has the tapes from the Number of the Beast and Piece of Mind rehearsals at home.... Apparently he invited the guys from Wheetus, who are huge Iron Maiden fans over and they got to hear the tapes, around the time of their hit Teenage Dirtbag.
Edit: Apparently Bruce's cancer treatment resulted in loss of taste, particularly sugar. Bruce can't taste sugar at all (and subsequently most fruit tastes like shit), but can make out some taste out of chocolate.