Conversation so far:
What was the Mirror challenge?
challenged Houdini to escape from special handcuffs that it claimed had taken Nathaniel Hart.
Who challenged Houdini?
London Daily Mirror newspaper.
Who was Nathaniel Hart?
a locksmith from Birmingham.

Question: What was special about the handcuffs?
Self-contained rewrite: What was special about the handcuffs that Nathaniel Hart made?