The formula exactum ad Castoris was intended to be an official guarantee of the standard weight of the piece. The word Castor or an abbreviation of it often appears on sets of weights because the temple of Castor in Rome was a fiscal depot and a sealer's office for adjusting weights. The 'S' in the centre of the inscription denotes the value, semis, (half) which equates to half a Roman pound or libra, today's equivalent of 6 ounces. For a detailed discussion of Roman and Byzantine weights, cf. B. Kisch, Scales and Weights, New Haven and London, 1977, pp. 150-155.