Carved in 1631, nearly two centuries before the Market House existed, it is a piece of heraldic stonework that has simply survived by being moved and reused, embedded into the masonry like a calling card from a world of crests, cadency marks, and impaled shields that most passers-by will walk straight past.

The plaque is a carefully executed example of seventeenth-century armorial display. A rectangular shield with an ogee-shaped base, that is, a curved pointed foot typical of the period, carries two coats of arms side by side, a practice known as impalement, used to show the united arms of a married couple. On the left, or sinister side, are the arms of a woman with the initials M C, featuring a bend of ermine charged with five roses or similar flowers, flanked by two saltires botonny, crosses whose arms end in small rounded buds. On the right, or dexter side, sit the arms of the Sherlock family, described in heraldic terms as per pale argent and azure, two fleurs de lys counterchanged, meaning the colours of the shield and the lilies swap across the central divide. A crescent mark differencing the shield identifies its bearer as a second son. The male party carries the initials I S, almost certainly a member of the Sherlock family, and the date 1631 is raised clearly in the stone. Above the shield, a gentleman's helmet supports a crest of a pelican in her piety, the traditional image of a pelican feeding her young with blood from her own breast, long used as a symbol of self-sacrifice and devotion. A matching coat of arms bearing the same initials survives in a wall monument in nearby St. John's graveyard, suggesting these two pieces were originally part of the same commemorative programme, perhaps marking a marriage, a death, or both.