The Shackle Opener (or Shackle Key) is used to open and tighten screw-pin shackles used on marine rigging. Available on some Marine focused 111mm Victorinox knives, the Shackle Opener is combined with a Marlinspike, that is used to splice rope, untying knots, and other rope work.
See also: Marlinspike - a different Marlinspike/Sacking-needle tool.
Wenger as several Marine focused 120mm knives in the New Ranger series that include a Shacke Opener/Marlinspike tool that is very similar to the Victorinox Shackle Opener. The Wenger tool includes both inch and cm scale marking on one side of the tool.