"We noticed that the year on it was '16. So we thought it was 2016," Glen told CNN. "Then we noticed that the stamp was a king rather than a queen, so we felt that it couldn't have been 2016."
The letter was addressed to "Mrs. Oswald Marsh" and was made out to "my dear Katie." The sender was identified as Christabel Mennel.
"Once we realized it was very old, we felt that it was OK to open up the letter," Glen said.
Glen recently took the letter to the Norwood Society, a local history group that publishes the quarterly Norwood Review.
"It's very unusual and actually quite exciting in terms of giving us a lead into local history and people who lived in Norwood, which was a very popular place for the upper middle classes in the late 1800s," Stephen Oxford, editor of the Norwood Review, told the BBC.
Mennel wrote in the letter that her family was vacationing in Bath, England.
It remains unclear why the letter took so long to arrive at its intended destination.
"We appreciate that people will be intrigued by the history of this letter from 1916, but have no further information on what might have happened," the Royal Mail said in a statement.