A locked ball and chain was yesterday unveiled as one of the most unusual items to have been pulled out of the Thames mud in years. The question is: what happened to the prisoner?

The shackle, beautifully preserved in the foreshore mud, went on temporary display at the Museum of London Docklands today, where Kate Sumnall, the museum's finds liaison officer, admitted its full story would never be known. "The tide is very good at sorting things so if there were any bones associated with it, they would have moved," she said.

The ball and chain was found in a Rotherhithe barge bed by an official mudlark – there are many amateurs but only a few have permits to actually dig in the Thames foreshore – called Steve Brooker. "I almost left it there to be honest because for three weeks in a row I'd had cannonball after cannonball and this looked like another one," he said.

Sumnall said the find was incredibly rare. All that is known so far is that it is 17th to 18th century, and it weighs eight kilos. It would have been used for transporting prisoners and nothing to do with slaves, and it was probably made in Germany.

After being displayed at the museum it will return to Brooker who plans to hang on to it. He called the Thames "a lost world".