Swell added to the bell to improve timbre of low B, C, and D. With the straight version of the extension these notes sound muffled.
Recommend printing with a raft, wall thickness of at least 1.2 mm, and infill at 0%.
I used PLA. Other filaments are likely fine as well.
Demo video:
https://youtu.be/8I5ej5xXZ_c
The author marked this model as their own original creation.