As a former business colleague once said "I remain confused but after this clarification my confusion is at a higher level". Now I think I know what this was meant to say but I am not sure if this says it.
Red is new. Strikeout is removed from old.
9.4. Vehicles may not be fitted with torque biasing
differentials in full and free operation between the
driving wheels (It is not Permitted to fit any form of
differential that was originally offered by its
manufacturer as torque biasing, whether fitted in
modified form or not) unless:
9.4.1. A differential was not fitted by the manufacturer
9.4.2. The vehicle is restricted to running in a
nominated Class.
10.12.4. Cars in Class 1 may fit replacement
differentials, crown wheels and pinions provided the
differential action remains free operating
2012 Blue Book is available at http://www.msauk.org/site/cms/contentCategoryView.asp?category=609