Pseudoisidore Cluny

In 1963, Horst Fuhrmann argued that three copies of the A1 recension of the False Decretals are so closely related that they must go back to a common source, and suggested that this was a copy made in Cluny around the year 1000. The oldest of the three codices, namely Paris, BnF, nouv. acq. lat. 2253 (saec. XII), was written in Cluny. For this reason, it is best known as the Cluny version of Pseudoisidore, or as Yale version on account of the oldest manuscript (New Haven, Beinecke, 442)

Indeed, with the discovery in 1970 of this manuscript, a mid ninth-century copy, the Cluny recension has become much more important for understanding the Pseudoisidorian forgeries. As Knibbs put it (https://pseudoisidore.blogspot.com/2010/03/introductory-iv-varieties-of-pseudo.html?m=1):

Until the 1970s, everyone thought that this was one of the later, unimportant versions. But then New Haven, Beinecke Rare Book and Manuscript Library, Ms. 442 turned up ... It now looks like this manuscript, from the mid-ninth century, gave rise to all the later medieval copies of the so-called Cluny recension (some of which ended up at Cluny, which is why we have this daft name for it). This book is exciting because it looks like it was assembled in the Pseudo-Isidore's workshop, perhaps even at Corbie.

The Cluny version (to judge from Schon's description of the Yale manuscript) contains parts 1 and 3 of the False Decretals, but with some omissions, namely the De primitiva ecclesia tract and the Constitutum Constantini as found in part one of the A1 version (see the overview here). The letters of Leo I found in the Cluny version is "Collection 21" in the Ballerini's list.

Fuhrmann, Pseudoisidor in Cluny (online); Schon, Redaktion (online).