Courtney Finn, who chairs Grantham Civic Society, said they featured an unrelated family from the town with the same surname.
He said it may be that whoever sent them believed he was related.
The collection includes correspondence sent from Gallipoli and records of family holidays.
Mr Finn said: "We think someone was clearing out their loft, found these photos and very kindly posted them to us in Grantham."
He said the postcards and photographs are dated between 1905-1932, and feature George and Alice Finn, who lived at 91 Stamford Street, Grantham, and are buried locally.
However, he said: "It's a real mystery why someone sent them to us."
"I've even been and knocked on the door in Stamford Street to see if anyone remembers them, but it's all too long ago," he said.
One postcard sent from a friend or relative serving in Gallipoli said: "Took over three weeks to get here and rather hot too."
Another said: "Keep a loaf of bread - meet us at the train station."
Mr Finn said: "They are a magical reminder of all those years ago."
The historian said the parcel was postmarked Leighton Buzzard, Bedfordshire, and cost Â£6 to send.
He added that someone had gone to a lot of trouble and is urging them to get in touch.
A historian is appealing for help to solve the mystery of a 100-year-old collection of postcards and photographs sent to him anonymously.