Just as a note, I play (very occaisonally) the whistle, mostly a black Plastic (ok there is a fancy name on the material used) susato with removable head, beautifull, but also own a Shaw low d. Even though the Low D did take practice to stretch the fingers to the bottom holes, I can do so ok. However i have tried a susato low D, a clarke low D and just cannot manage them, whatever the slight difference is, it's just the make or break. I also have a shaw bog standard D whistle wich i hate, so it's resided in a jam jar for the last few years.
Just shows it's a matter of personal comfort i suppose.

Anyway Susato shouldn't have released an inferior instrument, good luck with your quest!