Collectio Pauli
From Clavis Canonum
This collection of 53 letters of Gregory the Great was compiled by a certain Paulus (at Corbie), and Ewald refers to it as the "Collectio Pauli" or P for short. He knew five manuscripts containing P but no other collection of Gregory's letters, 14 that combine P and C (the Collectio CC epistolarum). For combinations with ρ, see Registrum Gregorii.
Ewald, Studien, https://resolver.sub.uni-goettingen.de/purl?PPN345858530_0003