The Collaborative International Dictionary of English v.0.48:

Moldboard \Mold"board`\, Mouldboard \Mould"board`\, n.
   1. A curved plate of iron (originally of wood) back of the
      share of a plow, which turns over the earth in plowing.
      [1913 Webster]

   2. (Founding) A follow board.
      [1913 Webster] Molder

WordNet (r) 3.0 (2006):

mouldboard
    n 1: wedge formed by the curved part of a steel plow blade that
         turns the furrow [syn: moldboard, mouldboard]