The ordinary "plug hat" was known to these people, but not an opera hat.
He had told her that the box was hidden in an opera hat, in one corner of the room.
When he had gone I went into my bedroom and made a pile of my opera hat.
He was in evening dress, and an opera hat was set rakishly on the back of his head.
He had felt uneasy with the very servant who gave him his opera ...