I have a pair of Visaton 1-inch throat adapters for their Smith horn.The round inlet is 25mm (1 inch) diameter, measured with a scale.
The outlet is 42mm (1-5/8 inch) horizontal by 32mm (1-1/4 inch). vertical.
The transition distance is 70mm (2-3/4 inch) face-to-face.
If you use the Westlake-style adapter this won't make any diference, of course.
FWIW,
Charlie