@TrueToneGoldWash On my collection of Bueschers - a #12 with Bb to A rotor valve, a #9 and a 243 - that they work best with cheapo Arnold & Sons Bach clones if you shorten the shank by about 4 millimetres and take off the outside silver plate on the shank - or use an Arnold & Sons cornet mouthpiece with a suitably altered converter. And Arnold & Sons mouthpieces (sometimes marketed as Stoelzel) are reliable clones of good Bach mouthpieces, at ridiculously low prices...

https://www.thomann.de/gb/mouthpieces_for_trumpets.html?manufacturer[]=Arnolds %26 Sons&gk=BLBMBT&sp=solr&category[]=BLBMBT&cme=true&filter=true

https://www.thomann.de/gb/mouthpieces_for_cornets.html?manufacturer[]=Arnolds %26 Sons&gk=BLBMBK&sp=solr&category[]=BLBMBK&cme=true&filter=true

Unaltered Arnold & Sons mouthpieces work very well wth all my other horns... Courtois Balanced, Olds Recording, Gaudet C, Courtois D...