Distinguished SOTW Member
I had a similar situation with my Hohner (Similar to your stencil). Making sure the cork on the very end of the neck was a snug (SNUG - VERY END) took care of most of it. I had been dealing with it old school - it's me and not the horn - but when I played Chicago there was a lick that was unplayable because of this problem. After a week of intense leak checking and long tones I used teflon tape to make a tight fit between mouthpiece and cork. By the end of the run of the show I had it down to the very end of the neck and the mouthpiece being the culprit. The seal of the cork to my mouthpiece was air tight, but there was space at the end of the neck where the cork was undersized relative to the internal dimension of the mouthpiece. This was somehow interfering with only low C.