There two types of shanks for horn mouthpiece. One is 3/100 taper, another is 5/100 taper. Generally, a 3/100 shank is called European shank and a 5/100 shank is called American shank. Yamaha horns use American shank mouthpieces.
The mouthpipe of Yamaha horns have a double shank, having both tapers (3/100 and 5/100) in a pipe. One exception is the YHR-868GD, which comes with two mouthpiece adapters, one for a 3/100 shank and one for a 5/100 shank taper.
Since some areas of the double shank mouthpipe make contact with both types of taper, the mouthpipe will fit both mouthpieces.
Do you need an additional help?