Piston perhaps a study of the rules would reveal that "any frame" is referring to a converted "road bike" frame of that era. Prior to 1973 most MX sidecar frame cradles were either modified road bike or one off hand made specials as factory frame makers were even rarer than now!, also there is/was a list frames that excludes some later factory designs for Pre75 in the MoMs. Please be remembering the set limitations on suspension travel and shock types for Pre75 and pre68 , this is the area that needs to be scrutinised much more thoroughly. I use a stock 1971 model CB750 frame cradle and swing arm ,having only extra road fitting brackets removed. To that I have now re engineered an early Wasp front end to fit up to the standard 754 head set. This is also fitted with a heavy CB750 disk brake. Cheers Tim754
Manual of Motorcycle Sport 2010, pages 169/170 starting at 18.9.0.1 deals exclusively with VMX sidecars and of course any general suspension /machine regulations for Pre75 were applicable.