Some older Wenger Swiss Army knives include a shackle (sometimes called a bale or bail). It's a metal loop on the end that can attach to keys, a lanyard or a pocket watch chain. Shackles were popular in the mid-1900s. In the 1970s shackles were gradually replaced by key rings or key chains (which are in turn giving way to carabiners and pocket clips).
Uses: If you decide to carry a vintage knife, you can use the shackle to attach your pocket knife to a couple of keys, to a larger key ring, or to just about anything else.