The symbol references Spanish currency. The S stands for Spain and the two vertical bars are the Pillars of Hercules an icon of Spanish royal heraldry and a prominent feature of Spanish currency.

During the colonial period and especially during the American war of Independence and following the 13 colonies-USA were short on specie currency and did not yet have a standard. During this time Spanish pesos, pieces of eight, were both common and of well known value (their precious metal content was known by weight and size). As a result the peso was a very common currency in transactions or as a reference currency for the many other currencies in use.