[Graham]

Are rigid fibreglass bodyshells still allowed as there's no mention of them in this version of the rules? 

And what's the difference between Class A car with B&W chassis and a Class C car with B&W chassis?

Yes I have included the Fibreglass bodies, it was mention under bodies section but not carried over into the Class A definitions.
Class A allows Resin or Fibreglass bodies to be fitted with B&W or metal chassis because the Cast body weight is generally heavier than Mass produced bodies of Class A.
Class C Procars are by definition those with lighter mass produced bodies (usually around 5gms lighter) than their home based RTR body equivalents. And similarly 3d printed bodies by the nature of the process are lighter than their Cast body equivalents.