What is a PS1 trumpet backbore?
Edited
The Pickett PS1 trumpet backbore is a design based on the Blackburn short piccolo shank mouthpieces and have a trumpet size shank. The PS stands for "piccolo short shank".
The Piccolo Short Shank Blackburn mouthpieces were initially intended to work with the Blackburn piccolo trumpets, where the backbore was designed as an extension of the piccolo leadpipe. This led to much improved intonation and playability.
The PS1 backbore comes in a whole host of throat sizes like all Pickett backbores.
PS1/27
PS1/26
PS1/25
PS1/24
etc.