All mouthpieces now have a standard taper size, depending on whether they are little, medium and large shank. I will give you some measures so that you can determine what is causing the problem.
Inner diameter of the large size receiver end: 13.9mm (+/-) 0.1; shaft end outer diameter of DW SM / K&G mouthpieces 12.8mm and fit into receiver for 22mm (stem length 36mm); Schilke 51D mouthpiece, at least 30 years old: 41mm stem length, 12.5mm stem diameter, fits 27mm into the receiver.
2007 Besson Prestige 2052, 3D+ K&G mouthpiece; JP373 baritone, 4B modified K&G mouthpiece; Bach 42GO trombone, T4C K&G mouthpiece; 1973 Besson New Standard 3 compensated valves, 3D+ K&G modified mouthpiece; Wessex French C tuba, 3D+ K&G modified mouthpiece.