Perhaps I should clarify what I mean by the ribs

Looking at Chance Additions, p.40-41 I have shown what I mean by 'vertical' ribs in image 4. 'Standard' ribbing is where the ribs go along the length of the rest. The rests can sometimes be narrower than normal, and I'm sure this defines the much earlier (scarcer) models.
Sorry to have caused a possible confusion... or have I exacerbated this situation
