Major Brett Warner was at Camp Rhino in southern Afghanistan in 2001 when a US soldier stuck his head into his tent and asked if he would like to send a letter home.
With no paper in sight and just five minutes to spare, he tore up an empty ration pack box and quickly made three postcards for his wife and children.
The stranger put the cards into his pocket as he left the base and Major Warner soon forgot them.
"I had no idea if he was actually going to send them but he stuck to his word and when I got home there they were up on the mantelpiece," he said.
The cards are now part of an exhibition at the WA Museum that gives a glimpse into the secret world of Australia's Special Air Service Regiment.
The world-first exhibition titled Out of the Shadows has been put together to mark the regiment's 50th anniversary.
Among art hanging in the museum is a painting by Ray Reeves called SAS Soldiers At Rest, which depicts life at a base camp.
Major Warner said the Australian flag always flew at the camps.
Perth artist Ian Coate, who served as an army illustrator- reprographic artist, contributed more than 20 pieces.
The free exhibition is on at the WA Museum until June 1.