I look forward to the clarification Col. The problem as I see it is that chapter 18 is exactly the same as chapter 17 and 20. That is, none of those chapters specifically require a chain cover to prevent entrapment. However at a local club day here last weekend, the scrutineer carefully explained to me why the cover must be in place, and it is due to the operation of chapter 12. If that is the case, then it applies to all disciplines, which includes classic MX as much as modern MX and DT.
If Dave is right that chapter 18 by specifically NOT mentioning rear sprocket covers somehow prevents the operation of chapter 12, then the same applies to modern MX and DT, as their chapters similarly do not specify anything about covers. Which means our scrutineer is quite wrong to be enforcing that requirement on vintage bikes racing at modern meetings (ie our local club's classic class).
Therefore I can see no argument, based on Dave's interpretation, for MA to enforce rear covers on any Motocross, Dirt Track, or Classic MX/DT motorcycle. I will be passing this question on to MA for an adjudication (however one does this). Chapter 12 says what it says and it is meant to apply to all disciplines. Classic MX gets no out on this one unless it is specified somewhere in Chapter 18 which it clearly isn't. And Chapters 17 and 20 are in the same boat.