LONDON — Stephen Morris, a professional musician, was so exhausted after a long day in the recording studio that when he got off a train in southeastern London on Oct. 22, he did not realize he had left his 310-year-old violin behind.

“Devastated” once he realized his mistake the following morning, Mr. Morris started a treasure hunt for his missing instrument — one of the few made by the master craftsman David Tecchler in 1709. It is said to be worth about 250,000 pounds, or more than $320,000.

He wrote to Southeastern Railway, which operated the train he had taken that night, and made public appeals on social media for its return. The British Transport Police later released an image taken from CCTV of a man who might have taken the violin , asking him to get in touch, British news outlets reported.

As time passed, the possibility of its return seemed increasingly bleak. But on Saturday, Mr. Morris announced a surprise breakthrough: His antique instrument was back.