An Australian family heirloom has been recognised as a rare piece of trench art and gifted to a French war museum.

The copper plate sign was brought back from the Battle of Fleurbaix by Private Victor Offe.

On the 100th anniversary of the battle, Offe's grandchildren Paul Fullston and Victoria Petho returned to France with the sign.

Unsure if it was a local town sign or an Australian-made souvenir, the pair presented it to the Museum of the Battle of Fromelles seeking more information.

Museum director Claire Fillassiez gasped as they presented her with the sign, Ms Petho recalled.

"She saw it and went: 'Oh! Trench art!'

"It was thrilling not just knowing the outcome but to think it had some value to the museum itself."

The grandchildren also gave the museum one of Offe's personal diaries and a letter describing the battle.

A letter home from Victor Offe on July 29, 1916 describes his experiences in the Battle of Fromelles. ( 891 ABC Adelaide: Brett Williamson )

"She [the museum director] said: 'Are you very sure you want to give this to us? This is so personal, so special'," Ms Petho said.

"They could really not believe that the letter existed and that we had come all of this way and were prepared to hand it over."

The value of the items were seen to be of such great importance to the area, a ceremony involving the mayors of Fromelles and Fleurbaix to mark their handover was held at Pheasant Wood Cemetery.

"We will never forget it," Ms Petho said.

"People near us had tears in their eyes."

Private Victor Rudolph Offe, a machine gunner. ( Supplied )

Ms Petho said she was glad they had travelled to France to give the sign back.

"We think that [Vic] would think it was a really great idea," she said.

"We have no regrets or second thoughts at all."

The family are planning to also donate Offe's Australia shoulder badge, his rising sun badge and other personal items to the museum's collection.