PC - A "117" style backbore tapered to fit piccolo trumpets designed to have a cornet sized shank. ie Schilke

PT - A "117" style backbore that tapered to fit piccolo trumpets designed for a trumpet shank. ie Blackburn

Please note: 
These backbores are designed to have the mouthpiece at the length of a cornet mouthpiece.
The throat bore on all modular backbores is a #27

If you prefer playing Piccolo with a 'Standard' Trumpet mouthpieces overall length, the Modular #11 Trumpet Backbore is recommended.