Hi Torsten,
Thanks for the confirmation. That obviously explains the observed behaviour very simply.
Not intuitive to me. I would have thought the stencil was rotated in this case about its axis of symmetry. But I guess lily[ond does not really know the item, just a stencil, is a 'circle' with special geometrical properties. But once one knows about this, it's all very clear. Indeed. as I thought, lilypond has not the mathematical nicety of Postscript in this particular area (indeed, why should it? It is my mistake to think that.).
Andrew