A key which opened the doors of an 11th Century tower has been returned almost 50 years after it disappeared.
The brass key to St Leonard's Tower in West Malling, Kent, was sent along with an anonymous note to English Heritage.
The mystery sender, who said they "borrowed" the key in 1973 wrote: "Sorry for the delay."
English Heritage properties curator Samantha Stone said the sender was not in trouble and hoped they would get back in touch to give more information.
The key - thought to have been made at some point in the 19th Century - still fits in the keyhole of the doors to the Norman tower keep, although it no longer rotates.
Very little is known about the history of the building or its original purpose.
Some believe the tower was once part of a castle, built by Gundulf, Bishop of Rochester, while others say the builder was Bishop Odo of Bayeux, the half-brother of William the Conqueror.
Speaking to BBC's Today programme, Ms Stone said: "There is no evidence of latrines or fireplaces, or anything that would suggest a domestic purpose, so its purpose has always been slightly mysterious.
"We assume it was for the administration of the bishop's manor.
"So the mystery of the key is very fitting in the wider history of the tower."
She said the sender had "kept it safe all this time, which shows some care and dedication" and English Heritage was "very grateful" and intrigued by the unusual story.
She urged the sender to get back in touch, saying she would like to offer them English Heritage membership.