Return the back-pointer to  markings that
        may have been attached via Django's content type mechanism.

def marking_thru(self):
        """
        Return the back-pointer to  markings that
        may have been attached via Django's content type mechanism.
        """
        self_django_type = ContentType.objects.get_for_model(self)
        return Marking2X.objects.filter(content_type__pk=self_django_type.id,
                                        object_id=self.id)