An enamel sign that has been in place since before the second World War has gone missing from a watermill near Pontllanfraith.
David Constable, who leases Gelligroes Mill from Caerphilly County Borough Council, said he first noticed the sign had disappeared in April.
Mr Constable, who believes the sign has been stolen, said: “It’s outrageous that it’s been taken. It’s incredibly important – it’s a part of our heritage.
“It doesn’t belong to me – it belongs to everyone who walks past.”
“I can’t believe it’s been stolen.”
Mr Constable said he has seen similar signs go for hundreds of pounds on websites such as Ebay, and has offered a £50 reward to anyone who can help him find the sign.
The sign has been described as being around 30in by 20in in size.
Mr Constable, who works as a candle maker, restored the mill in 1992.
The mill has an unlikely connection with the sinking of the Titanic in 1912.
It is the location where Artie Moore, an amateur radio broadcaster, picked up the SOS signal sent from the Titanic more than 3,000 miles away.
A Gwent Police spokesperson said: “Anyone with any information can contact us on 101 quoting 2000146463 or you can direct message us on Facebook or Twitter.”
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