Oooops. Hit the wrong button too soon; the last post is me.
It seems you've not familiar with how the "non-locking" shackle on your fetter works; hopefully the foregoing pictures will illustrate it. Usually, it's called a puzzle shackle, although it would probably be more accurate to call it a cam lock. As you can see, one link passes though the other as a cam, and then the chain passes though it to hold the whole thing in place.
If the lock is permanently attached to the chain, then the cam link has to be large enough to thread the open lock through.
While I can't tell for sure from you pictures, judging from the design of the locking shackle, in all likelihood your fetter was made in the Middle East and may have been used not as a human shackle but as a camel hobble, as you can see here: