Click to see the route I took.
It has some extremely nasty looking jagged edges in the top (the bit between 18 and 30 cm) which were designed to fold down and then snap onto the leg of some poor animal. Quite what is a mystery to me. It's too big for a fox and too small for a bear. Badger maybe but badgers are far too clever to fall for this sort of trap.
It is extremely rusty (which is why I didn't pull the trap bit apart) so I guess it could have been there hundreds of years. Mind you in Cumbria we have so much rain it could have been there just a few months...