The collar consists of a series of the letter S in gold, either linked together or set in close order, on a blue and white ribbon. (See Collar Of S.S.)
“On the Wednesday preceding Easter, 1465, as Sir Anthony was speaking to his royal sister, on his knees, all the ladies of the court gathered round him, and bound to his left knee a band of gold, adorned with stones fashioned into the letters S.S. (souvenance, or remembrance) and to this band was suspended an enamelled Forget-me-not.” —Lord Lytton: Last of the Barons, bk. iv. 5.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894