The mailbox was buried in the ground and used by secret agents to relay messages to each other. A rock was used to cover it on the ground.
Many foreign finds were laid out in the museum, but Anders Thornberg, the head of Säpo, said his organisation had to be careful in selecting what to put on public display.
"There are still things we have to keep secret so that an opponent does not find out how we work," Thornberg says.
Despite the selective approach, Thornberg said that Säpo has gone from being a "closed" organisation to being "very open".