That is a Dr. Goodwin-style tang sight, 2 people are currently making one - Rex Holbrook and ron Snover. And yes, it might just be a requirement to get the sight low enough for short-range shooting...
This is the better one (it has the vernier plate), the BL model from Rex Holbrook:
This one has minimal interference with your thumb, as it is attached from the other side - and has adjustable angle (the normal one doesn't)
Although maybe the Lee Shaver Pedersoli Gibbs soule sight might just work - if the barrel doesn't interfere with your thumb.
Image courtesy of Buffalo Arms website...
This issue is finding a sight that will come down low enough, and at the same time not interfere with your thumb, which can be quite delicate indeed... FWIW, here a pic of my match martini in #2 Musket with the 'ordinary' holbrook tang sight; sight set for 78 meters - my thumb barely clears the little screw on the side of the sight, if you have meatier hands, you just might be unlucky: