Watch List notes:

Elizabeth II, proof sovereign, 1974, diademed bust r. by Arnold Machin, rev. St. George slaying the dragon, reeded edge (S.4204), in plastic holder, graded by PCGS as Proof 67 ULTRA CAMEO, a nearly perfect specimen exhibiting a marvellous portrait as well as full proof characteristics, accompanied by a copy of a letter from the Royal Mint attesting to its proof status - a major modern sovereign rarity for the finest of collections
£12,500-15,000
This is a new discovery – it appears to be the only example known as a proof.