It is part of a system of marking the cannon to identify such things as the foundry that made them. The ends of the pivots (formally called trunnions) are a popular place to mark, since they will usually be visible. The markings can be very helpful in identifying the history of individual cannon.

The Royal Armouries website provides information about 18-pounders at Edinburgh Castle here, and includes a note that "The bowling Ironworks of Bowling Manor, near Bradford, began trading in foundry and smith's work in 1784. In 1788 largely due to the efforts of the ironmaster John Sturges of Wakefield, they started smelting iron ore in quantity. The company supplied the Board of Ordnance with cast-iron guns and mortars throughout the Napoleonic Wars, the letter S stamped on the trunnions being their identification letter." [emphasis added]