Hello,
as usual, I’m happy to share a little piece of information that could be useful to Susato whistles players.
In addition to the O-ring trick, mentioned here:
https://forums.chiffandfipple.com/t/mtguru-o-ring-susato-tweak/47141/1
I found that a pea-sized ball of poster putty, properly placed on the labium ramp, alters the lower or upper harmonics quite effectively. You can make the upper notes much more manageable, or render the whistle less recorder-ish. You’ll have to experiment, as it all depends on the geometry. My favourite configuration leaves the bottom notes alone, and makes the upper notes quieter.
There you are a couple of pictures:
http://guido.gonzato.googlepages.com/Susato01.jpg
http://guido.gonzato.googlepages.com/Susato02.jpg
My suggestion is that you make a drop-shaped blob of putty, then apply it on the labium ramp with the tip pointing towards the mouthpiece. Start from here, then change the geometry of the ball and hear what happens.
Ciao,
Guido =8-)