To continue my current Heckelphone obsession...
If you ever wondered what one sounds like, Paul Hindemith wrote a trio that features it. I bought this recording in order to obtain copies of his oboe and English horn sonatas, and the Heckelphone trio happened to be one of the selections on the album.
It is an extremely rare instrument, not even being made any more. However, a few people have managed to find one of the hundred or so remaining in the world today, and taught themselves to play it.
There is no one who teaches it.
There are no fingering charts.
The player must make his/her way alone. Since it's essentially a bass oboe, having oboe and English horn experience would help. Being a bassoonist, not so much, as the embouchure is different, and so is the method of tone production.
Hot-diggety-dog! Look what I found on You Tube! Here is one movement of the Hindemith Trio!
I think my oboist character "needs" one, to add a further dimension of classical music geekery to my already-plenty-geeky story...