Thanks! Bryan Loyd’s method of casting to an interface seems to be what I am looking for.

However, I get an error in combination with the commercial API:

>       surfaceCB = ZOSAPI.Editors.LDE.ISurfaceCoordinateBreak(surface)

E       TypeError: object does not implement ISurfaceCoordinateBreak

I will try to get a further hint from the support of the commercial software I am struggling with.