This is one of the most spectacular mint error mules of all time. The obverse of the regular design of the George IV Bare Head Half Sovereign was paired with an 1823 pattern reverse.
No 1829 Half Sovereigns were ever struck. It is one of two known. The other piece is impounded in the Royal Mint Collection. It is also a Dual Date with different dates on obverse and reverse. No other gold mules are known, mint state or proof.
This is an extreme rarity with a great history and pedigree. It is comparable to a U.S. numismatic rarity that sells for high six figures or more with only a few known and one impounded in the Smithsonian.