That's why the Farrer resident is so desperate for its return that he's offering $500, no questions asked, to anyone who can return the M15 Adrian steel helmet.

The combat helmet was among a box of possessions Mr Dopita's mother gave away at the St Vincent de Paul store in Queanbeyan about five months ago, around the time Mr Dopita's father died of liver cancer.

He said the piece of armour was "completely irreplaceable", and credits it for potentially saving his great-grandfather's life.

Mr Dopita's great-grandfather Alfred Bernard was wearing the helmet when he was shot while serving with the French army during World War I.

Mr Dopita said the bullet had hit the joining rim around the helmet, making the bullet ricochet off at an angle and preventing it making a bigger hole.

"[The bullet] still did some severe damage to him and he had a huge scar and apparently a few mental issues after that, but he was still able to have my grandma," Mr Dopita said.

"Obviously that’s where the chain that leads to me really comes from.

"It’s completely irreplaceable. I could buy the same sort of helmet, but it wouldn’t be worth the same to me.

"It means the world to me and it’s the only item that my parents had that ties me to my family history."

Mr Dopita said he had only found out the helmet was gone this week, and he was especially disappointed for his three children, who he hoped to be able to pass the helmet onto one day.

He said he had been in contact with management at the St Vincent de Paul store, as well as antiques stores, eBay, Gumtree and the Australian War Memorial, but no one knew what had become of the treasured item.

Now, Mr Dopita has turned to the internet in the hope of being reunited with the helmet, posting calls for information on several community Facebook pages and collectibles groups across Canberra and the Queanbeyan region.

"I am offering a $500, no questions asked, reward for its return," he said.

"It’s just such a piece of history.

"This was the one thing that bound me to my past and gave me a feeling of belonging to something so much bigger."

Mr Dopita asked anyone with information on the helmet to email him at igor.dopita@gmail.com