The small indention is where you should align your lower lip when you blow.
Sizes are in US and diameters(mm). Pick your size and print. I've test printed in both PLA and PETG, both work great.
Smaller sizes may require a brim to properly print.
Enjoy!
updated 17.02.2022-Added 3mf file versions. I also was able to print in TPU. See under Makes and Comments.
The author marked this model as their own original creation.