Watch List notes:

WINDSOR. Elizabeth II. 1952-present. NI Trial Die Strike (25mm, 5.07 g, 12h). London mint. Dated 1957. Britannia seated slightly right, striking coins; three coins on ground before her; to right, dolphin upward / ROYAL MINT ~ TRIAL DIE ~, hexagram; 1957 (date) in center. Cf. DNW 135, lot 844 (for type). Some original color, crimping to edge. EF.

These 1957 Royal Mint trial strikes were reportedly found by metal detectorists in spoil heaps in Dagenham, East London where the remains of the Royal Mint building on Tower hill were taken after demolition.