The Studio Master FL4 has an inner diameter of .637" or 16.18mm.
Studio Master Flugelhorn models are available in 2 cup depths.
FLS - Shallow flugelhorn model This is a fabulously flexible mouthpiece. It offers rich sound with plenty of ease of execution for solo work. The Bore Size is a #22 (.157"or3.987mm) and the Cup Depth = .970" or 24.638 mm
FL - Deep flugelhorn model Is a classic flugelhorn design, a deep "V" shaped cup with just enough bowl to add an extra touch of darkness to the sound. The Bore Size is a #18 (.169" or 4.29mm) and the Cup Depth = 1.095" or 27.813 mm
Various manufacturers have different dimensions to the receivers on their flugelhorn instruments. The standard for the Studio Master line is the same as that used for Yamaha instruments. However...All models are available with the alternative larger "Y" taper found on Yamaha, Benge, Getzen, Blessing and Jupiter instruments or, with the French or, Cuesnon taper. All non-standard tapers must be requested specifically.