Curiously, in Beckensall's 2001 book, the diagram on p118 shows concentric rings, not penannulars, whilst the photo on p119, from the 1960s, does appear to show the ridges. I guess that if the ridge is narrow enough, it will weather down faster, so that it becomes lower than the circular ridges of the rings, hence on a rubbing, it would look as if there is no gap in the rings.