Watermark purposes - Tradução em português

No contexto abaixo, o trecho qual seria o sentido do trecho "have answered for watermark purposes" ?

Think only what the supposed testimony of watermarks on paper might gain from the Tarot card of the Pope, in connexion with the notion of a secret Albigensian patriarch, of which Mr. Bayley has found in these same watermarks so much material to his purpose. I am not a good judge in such matters, but the fact that every one of the Trumps Major might have answered for watermark purposes is shown by the cases which I have quoted above.