What is "wrest plank"

Wrest \Wrest\, n.

1. The act of wresting; a wrench; a violent twist; hence, distortion; perversion.
   --Hooker.

2. Active or moving power. [Obs.]
   --Spenser.

3. A key to tune a stringed instrument of music.

   The minstrel . . . wore round his neck a silver chain, by which hung the wrest, or key, with which he tuned his harp.
   --Sir W. Scott.

4. A partition in a water wheel, by which the form of the buckets is determined.

   Wrest pin (Piano Manuf.), one of the pins around which the ends of the wires are wound in a piano.
   --Knight.

   Wrest plank (Piano Manuf.), the part in which the wrest pins are inserted.