The remarkable story was unearthed for the documentary by Oliver Urquhart Irvine, the librarian, and assistant keeper of the Queen's Archives.
Even though the height of the Imperial Crown was slightly lowered for her after the death of her father, King George VI, its diameter remained the same. The trap door that led to the secret area still exists today.
Bruce spoke to InStyle about talking with Queen Elizabeth and his theory about why she might be choosing to speak publicly on this right now.
The Queen rarely gives televised interviews, but the programme comes amid a revolution of the monarchy over the past 12 months, whereby the younger senior royal members have made regular TV appearances and her grandson Prince Harry is set to Wednesday actress and divorcee Meghan Markle, who is older than him and a woman of colour.
But bizarrely it won't be the Queen - who spent most of the war at Windsor Castle - that reveals the biscuit tin secret, as she never knew of it herself.
"Fortunately, my father and I have the same shaped head, once on it stays (fixed)", said Queen Elizabeth II, aged 91 who on February 6 will have been the British head of state for 66 years, amply surpassing Queen Victoria's reign of 63 years and 216 days, between 1837-1901. I mean it's only sprung on leather.
"At one moment, I was going against the pile of the carpet and I couldn't move at all".
The Queen Elizabeth II, dressed in royal regalia, posed for this special Jubilee picture in the Throne Room of Buckingham Palace, after she had delivered the traditional Queen's speech at the 1976 official State Opening of Parliament. "He said, 'Do you realise you have the real Crown?' And I said, 'I do realise that".
"So there are some disadvantages to crowns, but otherwise they're quite important things", the Queen adds.
The stones, including the Black Prince's Ruby from the Imperial State Crown, were placed in the metal box and buried under a sally port, a secured entrance.
He also said how "an electric set of letters" from Sir Owen Morshead, the royal librarian, to Queen Mary, the mother of George VI, shed light on the mystery.
"The idea that his plume was put into the stone. on his helmet".
"It is sort of a pageant of chivalry and old-fashioned way of doing things, really", the Queen recounts of the day.