Clever keyhole

Exploration Stage: Traveling Circus
From: Mr. Black
Description: I need to collect all the parts of the door used in the example performance that became the alibi for the circus troupe members. I believe this trick has something to do with the theft at the cabaret. But how? According to this floor plan, the door must have had a special keyhole cover. Let's find that.
Completed: This keyhole cover seems to have been dug out of a rare ore. It is a very delicate item. Why do they need such a thing for a mere circus trick, even if it is a performance in which all members of the troupe perform? It's just for decoration, Diggs repeatedly mutters...