S.A.G.    “Saint Anthony Guide

One of the practices of those who are devout to St. Anthony of Padua is to entrust their letters to his care. St. Anthony became the special patron of written messages and the protector who watches over the safe delivery of letters because of a remarkable miracle. A merchant had gone from Spain to Peru on business. Although his wife wrote to him several times she did not receive any answer, so she decided to get St. Anthony to help her. With childlike confidence she went to the Franciscan church in Oviedo and placed a letter in the hands of the statue of the Saint, asking him to deliver it to her husband.  Later she returned to the church, and, to her great joy, found, in place of her letter, a reply from her husband.  With it were several gold pieces.  The letter may still be seen at Oviedo.  It was dated:  Lima, July 23rd, 1729, and states that the wife's letter was delivered by a Franciscan Father. Usually, those who invoke St. Anthony's protection in this way write "S.A.G." on the back of their letters or seal their letter with St. Anthony Guide seals.

(source:  http://www.homestead.com/missionbell/Anthonycustoms.html)