Inside was an old and decaying roll of film.
The contents were too damaged to be played on a reel, but since the discovery in March, Ms Cappa has managed to digitise and restore a few frames - enough to show that the documentary was of a key event in Bolivia's history.
The flickering black and white images show the execution of Alfredo Jauregui, who was convicted of killing former President Jose Manuel Pando in 1917.
The silent documentary, filmed in 1927 after Jauregui and three others had spent 10 years in jail, captures  the young man moments before his execution, smiling nervously at the camera.
With officials and spectators gathered at the scene of the execution outside La Paz, the firing squad takes aim and shoots. Jauregui slumps and two priests approach to pray over his body. Punishment duly meted out, the huge crowd heads back home down the hillsides.
The film fragments are shocking enough, but they are part of an even more dramatic story.
According to Bolivian law at the time, when three or more people were found guilty of the same crime, only one faced execution.
Their fate was sealed by drawing lots - in this case a black ball.
But there was another twist - it subsequently emerged Jauregui was in all likelihood innocent, as were the other three.
The film, the last work by Bolivian filmmaker Luis del Castillo, a photojournalist turned movie director,  was censored as soon as it was released and remained unseen for 85 years.
News cuttings from the time show the film was not banned because of its violent images, but rather from the fear that if it were seen abroad, it could expose Bolivia and its judicial system to international criticism.
There was also concern among Bolivia's European-descended elite that the people shown in the film were mainly indigenous men and women, which, in their eyes, would also damage Bolivia's reputation.
Jauregui, who was only 16 when Pando died, was caught up in the political machinations of the era.
Historian Mariano Baptista Gumucio, author of a  book called The Death of Pando And The Execution of Jauregui, told the BBC the four men, who were all related,  had been accused because Pando had died in their house.
They had killed Pando, founder of Bolivia's Republican party, as part of a Liberal Party plot, it was alleged.
But the version of events as told by Jauregui's uncle Nestor Villegas, who was also convicted of Pando's murder, was more a combination of bad luck and bad judgement.
According to Villegas,  Pando had dropped by their house unexpectedly on his way to La Paz. They were sharing a couple of bottles of wine he had given them when suddenly Pando had a stroke and died.
Panicking and fearing they would be accused of his death, they wrapped him in a blanket and threw his body over a cliff.
The corpse was found and a post-mortem examination indicated that Pando had died of natural causes, said Mr Gumucio.
But these findings were never revealed and a second examination, apparently ordered by the leader of the Republican opposition Bautista Saavedra, concluded Pando had died as a result of injuries inflicted by the men.
"The men were falsely accused by Bautista Saavedra..., who used Pando's death as an excuse to overthrow the Liberal government and take over the presidency," Mr Gumucio said.
The legacy of those events resonates to this day.
Elda Jauregui, Alfredo's great-niece and granddaughter of another of the convicted men, said the discovery of the documentary would help to finally prove the men's innocence.
"We have suffered a lot due to what happened. This film will speak for itself," she said.
Mr Gumucio also believes this film could make Bolivia re-examine its past and learn the truth about Pando's death and Saavedra's accession to power.
However for that to happen the film, which runs for some 17 minutes, must first be digitised, but the FCB - a private institution- says it cannot afford the $10,000 (Â£6,000) needed.
The organisation has asked other Latin American film institutes for help but has been unsuccessful.
Without financial help the FCB (Bolivian Film Foundation) warns that this vital piece of Bolivian history could remain unseen forever.
Editing by Liz Throssell, BBC News
When Carolina Cappa, a film researcher working in Bolivia, was invited to go though the  archives of an old cinema in La Paz that was being demolished, she came upon a battered, unmarked tin.