The Collaborative International Dictionary of English v.0.48:

Supposal \Sup*pos"al\, n.
   The act of supposing; also, that which is supposed;
   supposition; opinion. --Shak.
   [1913 Webster]

         Interest, with a Jew, never proceeds but upon supposal,
         at least, of a firm and sufficient bottom. --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

supposal
    n 1: a hypothesis that is taken for granted; "any society is
         built upon certain assumptions" [syn: assumption,
         supposition, supposal]
    2: the cognitive process of supposing [syn: supposition,
       supposal]