Open class cars have a minimum weight if they are both 4wd and turbocharged.  If they are only one or the other, or neither, then there's no minimum weight.  Production class cars minimum weight is the factory curb weight.  Group 2/5 has no minimum weight.

The way that it's written, it's only Production class where the book NEEDS to say something for it to be allowed.
Production class rules tell you what you can do, and you can't do anything else
Open and Group2/5 rules tell you what you can't do, and you can do anything else.