The Collaborative International Dictionary of English v.0.48:

Probator \Pro*ba"tor\, n. [L.]
   1. An examiner; an approver. --Maydman.
      [1913 Webster]

   2. (O. Eng. Law) One who, when indicted for crime, confessed
      it, and accused others, his accomplices, in order to
      obtain pardon; a state's evidence.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PROBATOR. Ancient English law. Strictly, an accomplice in felony, who to
save himself confessed the fact, and charged or accused any other as
principal or accessary, against whom he was bound to make good his charge.
It also signified an approver, or one who undertakes to prove a crime
charged upon another. Jacob's Law Dict. h.t.