From what I could see, there hasn't ever been a definitive answer from the House of Commons authorities on this, so I made a Freedom of Information request to the HoC (now published on their website) asking for an itemised list of the contents of both despatch boxes in the commons chamber, with the below response:
We hold a list of the contents of each of the two despatch boxes in the Chamber of the House of Commons:
- Multiple versions of the Oath and Affirmation in different languages (including English, Welsh, Gaelic, Cornish and Braille).
- New English Bible
- King James Bible (Old Testament)
- Welsh Bible
- Gaelic Bible
- New Testament Bible
- Various pieces of blank coloured card
I thought this was fairly interesting, as although religious texts from other faiths are available to members when taking the oath of allegiance, it seems that just the Christian Bible remains in the despatch box during the parliamentary session. I remain ignorant as to what the pieces of coloured card in the opposition box are used for.