Collectio CC epistolarum
From Clavis Canonum
This collection of 200 letters of Gregory the Great, known as "Collectio CC epistolarum", "Sammlung der 200 Briefe" or simply C is almost always transmitted together with the Collectio Pauli but, as Ewald demonstrated long ago, had a separate existence and is in fact very ancient. Ewald knew nine copies of C.
Ewald, Studien, https://resolver.sub.uni-goettingen.de/purl?PPN345858530_0003
