Gee! thanks for putting me on the spot Burr ;)

Firstly, it's obvious there isn't enough information to tell how the form is made up, but we do come across this situation a lot at work when we receive geometry which is made up using a spline or series of splines and we do have to simplify it into analytical geometry (simple lines and arcs) for the CAM guys, or else they'll have a grizzle, so it is possible to break it down, you can't expect an exact match but we do get it within 0.0005" (0.01mm) sometimes closer.

Without the actual geometry I traced the picture as close as I could and show in the attached .3dm a series of trimmed tangential arcs that I used to make up the shape, you'll see off to one side (as shown in the jpeg) the arcs and there corresponding center points in the same colour as the arc, so if you pick on one of the arcs, MoI's information panel will show the radius.



Is this what you meant?

Cheers