Can someone tell me about this horn?

[bbocaner]

If it really is an alto, your trumpet mouthpiece should fit pretty well. It won't work well, but it'd be a good fit test. If it's a tenor, a small shank trombone mouthpiece might work, although it might possibly be a little big and not go in the full 1".

Report back.

[Disposabledolphin]

Yeah the trumpet mouthpiece fits well but sounds strange

[John Morgan]

I stand corrected (I am actually sitting) on my surmising that the horn in question might be a baritone. Not too many trumpet mouthpieces that I know of fit in a baritone horn's receiver. So, I am going to stop guessing the type of instrument based on a picture. Instead, I am going to get on my riding lawn mower and do a final spin around the yard before the first blizzard. As for what mouthpiece you should be looking for, I do know the answer to this puzzler - "the right one".

[Disposabledolphin]

I think the trumpet mouthpiece goes in just slightly too far, but it isn't too far off!

[bbocaner]

I'd suggest a Dennis Wick Heritage Series Tenor Horn 2 or 3.

[LargoBone]

I agree, look for an authentic tenor horn mouthpiece, that's probably the right size.

Either way, F. Besson's are some of the most highly sought after/copied horns in the trumpeting world if that tells you anything. A gem!