The Collaborative International Dictionary of English v.0.48:

Contract tablet \Con"tract tablet\ (Babylonian & Assyrian
   Antiq.)
   A clay tablet on which was inscribed a contract, for safe
   keeping. Such tablets were inclosed in an outer case (often
   called the envelope), on which was inscribed a duplicate of
   the inscription on the inclosed tablet.
   [Webster 1913 Suppl.]