Victorinox

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 marlin spike, that is used to splice rope, untie knots, and other rope work.

See also:
Marlin Spike - A different marlin spike/sacking-needle tool.
Large Marlin Spike - A more recent thick marlin spike/shackle-opener.

Wenger

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.