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.