It's a long story. The short version: I'm building a hybrid instrument out of an old parts bassoon, and the bell key is missing. The bell is a Masson, and I'm told by Those Who Know that I need to replace it with a Masson key, as each manufacturer is different. Anybody know where I might find such a thing? Or does anybody have one just sort of laying around collecting dust? Thx.

As you've been correctly told parts are not usually interchangeable between brands but you might find something to fit if you can find someone with a good bassoon bone yard.

In 30years of interest in bassoons I've never heard of a maker called Masson. Maybe they were a store who bought in a few bassoons from a manufacturer marked with their own name - quite a common occurrence over the last few hundred years.

Good luck with your hunt.