Cup lock shackles
I have seen the Cup Lock the first time in the Medieval Crime Museum Rothenburg (Bavaria/Germany). A shackle locking this way does need neither padlock nor key but in first place a chain to secure it shut.
You close the shackle, push the cup shaped piece over the two flat parts, and then pass a chain through the aligned cross-shaped holes. The chain used may have a block at one end. Place a lock on the other end or secure it in some other reliable way.
Then you cannot pull the chain out of the hole, the cup cannot be taken off the shackle which in turn cannot be opened.
Nowadays one does not come across newly manufactured cup lock cuffs intended for sale. But I have been contacted by Blacky who actully has built such cuffs in his own private workshop and has been so kind to send me the photos you may see below.
I think this cuffs are well done. If you are interested in a few remarks and hints on how to still improve, I have written about it on a more techically oriented page.
Brief overview of different models:
These itmes shown in Rothenburg have round cuff openings and rectangular cup locks. One may note the chain link old style on the right specimen and the long shaft on the end chain link.
These cup lock shackles on display in the Emperor's Castle in Nuremberg have a rounded cup.
And two specimen from the dungeons underneath the town hall in Nurenberg. The rectangular cups are slightly different in their make. But more interesting is the fact of the left shackle being elliptical in shape and the right shackle being round. Oval shackles if fitting snugly can not be rotated around the limb, which may be an advantage because loosely dangling restraint can turn turn to be quite a nuisance.