I think this is the $64,000 question in the marble world. I have never been able to figure it out at all. Les Jones once told me that all machine marbles have seams. It's just that often we can't see them and this makes a lot of sense. I mean a gob feeder has to leave some kind of mark, doesn't it. Could it be that the seams get buried inside the mib, or postioned and shaped on the surface, as it is being formed ? I think so.