Clarin notes simply that the incident is known as the "mattress riot" (Motín de los colchones) because detainees set fire to their mattresses; around 60 of them died of asphyxiation and burns, while another 85 were injured, according to the paper.
Historias de fugas y motines sangrientos (Clarin)
In a 2005 article calling the riot "a tragedy", La Nación notes that the riot was sparked by an apparently minor incident; a prison refusing to turn off the television, having been ordered to do so by a guard. Other prisoners backed him up, and the following day (14 March) made barricades of mattresses and other flammable materials and set fire to them. They were trapped by the smoke and flames; the paper reports that there were 61 dead.
It also states that following autopsies, the prison service found that there was no evidence the victims had been killed by shots, but rather that they were burnt to death or died of smoke inhalation.
La peor tragedia, en la cárcel de Villa Devoto (La Nacion)
However, other sources suggest a very different story. In February 2013, for example, Daniel Enzetti wrote an article for Tiempo which claims that 64 prisoners were murdered by the State in Devoto. This report repeats the assertion that there was an initial arguement over television on 13 March, but then states that the argument was brought to an end, until state officials came in the next day with the intention of committing deliberate "genocide".
In this version of events, many of the victims were shot, the fire service was prevented from entering the building, and the crimes were covered up.
Some human rights organisations now want the case reopened. Supporters of this version call the incident the "massacre of pavilion 7".
Piden reabrir la causa por el "Motín de los Colchones" (Tiempo)
Una masacre que fue disfrazada de "motín" (Tiempo)
MASACRE EN EL PABELLON 7 (CEPOC)
Pagina/12 puts the number of dead at "at least 64" in one article and "at least 65" in another. It reports on efforts to instigate legal proceedings against the supposed perpetrators of the massacre, the heads of the prison service at the time and the officials present at the time when the deaths occurred.
Más que motín, una masacre (Pagina/12)
Masacre del Pabellón 7 (Pagina/12)
One of the few descriptions of the deaths in English appears to be from former political prisoner Margarita Drago in her book "Memory Tracks", although she remembers the incident taking place to be in July, not March. She writes,
One cold July day, from the solitary confinement chamber where I found myself being punished, I heard a series of gunshots which came from the vicinity of the prison, and I thought that the shots were directed towards the windows of the cells. [...] After a few hours, the matron opened the door to my cell, and without any explanation told me that I had to go back downstairs to where I normally stayed [...] Once in my cell, the women gave me the news. The prisoners had rebelled and burned mattresses in protest of their living conditions. The response was their wholesale murder. For this reason, they kept us incommunicado, forbidding visits and canceling recreation. In repudiation, we refused our food and declared ourselves in mourning.
In time, after they had lifted the sanctions, we learned that one hundred unarmed civilian prisoners had been massacred by their jailers. [p. 97-98]
Elias Neuman wrote a book on 14 March 1978 called Crónica de muertes silenciadas and now apparently out of print.
It would be irresponsible of me to comment on what I think is the "true" version, because I honestly don't know. Certainly, 1978 was an extremely violent period in Argentine history and a great deal of atrocities were taking place at this time; however, this event would be unusual since generally speaking, being a "common" prisoner was a much safer situation to be in than being a "political" prison, which could easily turn you into a disappeared person. Some (few) disappeared people survived because for one reason or another, they were transferred to normal jails and given a standard record as a common prisoner. Certainly, though, even these regulated jails had standards way below those which were either desirable or safe (almost all accounts mention that Devoto was overcrowded, for example, surely heightening the fire risk). What is clear is that even with deaths on this scale, even the most basic facts such as the number of victims are not really agreed on.