Carbony low whistle diameter?

[Tom-fae-the-Vale]

Could someone please advise what is the external diameters of the Carbony Low D? (I’m aware they’re tapered). I’ve puchased a thumb rest for my Low D (Chieftain) that’s actually advertised as suitable for use on ‘Carbony Low and Mezzo’s so I assumed it would also fit other Low D’s. It just fits on my Kerrywhistle Busker high D which is approx 19mm diameter. I didn’t expect the carbony Low D to be so narrow or, of course, have I been sent the wrong part… tia