The radius at the bottom of the "slit" is a lot less than the radius of the top of the original "flared horn" rim, and the two indentation have
stretched the "leather" quite a bit.
So maybe the original flared horn, before slitting and roll down, was circular, not with a raised up section.

- Brian

The flared horn shape can be the revolve of the appropriate curve.
Iso's would be available.

http://www.skywaves.ar88.net/xtal/horns/horns.html

The flare curve could be traced from one of the side views. The curve does not appear to be uniform.

Sometimes rounded shapes in a photo can look square-ish, but they may not be...