Wendy, I found this file, and an observation is that here "Cartas abiertas" is translated as "Opened Letters" and not "Open Letters", as per your text.

https://www.filepicker.io/api/file/em3gQDtCQVCEYJad9heW
De la periodicidad del tiempo de la propia revista se articulan nociones, como las del **tiempo no atendido**, que irrumpen en varias obras como *** Opened Letters [Cartas abiertas] (1972)***, que se presenta de forma documental como una carpeta llena de sobres de cartas recibidas. El proyecto surgió después de que el colectivo abrió su correo de varios días en el estudio y arrojó el contenido a la basura mostrando una cierta rebeldía antisistema y juvenil. Los artistas colocaron los sobres en una
carpeta, de acuerdo con la fecha de recepción, y los marcaron: “Recibido 1 de mayo, 1972, “Recibido 2 de mayo, 1972”... formando una línea de tiempo.

The frequency of the magazine itself gave rise to other notions of time, such
as unattended time, which appeared in works such as Opened Letters (1972),
presented in a documentary manner as a folder full of envelopes in which letters had been received. The project emerged after the collective opened several days’ mail in the studio and threw the contents in the garbage bin in a show of youthful anti-establishment rebelliousness. The artists put the envelopes in a folder, arranged by date of receipt, and marked them, ‘Received May 1, 1972’, ‹Received May 2, 1972’, and so on, forming a timeline.