Sentence:  They [turn-wrest ploughs] are now so constructed that the ploughman can readily shift his coulter by means of a lever, which reaches the bottom of the handles, and also his <WSD>wrests</WSD> or mould-boards from side to side, without leaving his station between the handles of his plough, they being so arranged that by withdrawing a small pin and pressing the projecting <WSD>wrest</WSD> towards the body of the plough, the mould-boards on either side become alternately the land side when not in work. 
Meaning of the word marked with <WSD> tags:  A metal (formerly wooden) piece of some ploughs attached under the mouldboard (the curved blade that turns over the furrow) for clearing out the furrow; the mouldboard itself.