Here is a third pz2 file. It uses empty geomCustom blocks to clear then geometry from sparePart-1 and sparePart-2, it also hides the actors and channels for these parts. It injects a torus geom into sparePart-3, and smartparents it to the BASEpart actor.

I have offten thought that this technique of using ghost actors and geometry injection could be a good way to impliment architectural and other modular items, but I have never taken it further than these few simple experiments. I guess the proof is in the pudding, perhaps some day I will take this idea further.