By the way the 2016 SSR's say nothing about a bike being British for class A. Just got to be manufactured pre1970.
I imagine this definition was influenced by the ACU requirements for trials, though, there are a variety of classes for a variety of trials in their handbook based on suspension type, rider age, etc. As an example, the ACU Sammy Miller Trial Championship has classes for pre-unit rigids, pre-65, British pre-70 pre-unit/unit/2-stroke, twinshocks, drum brakes, etc. If someone knows the background to the MCC SSR on this point, perhaps, they could advise

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The club rules aren't always logical.
But they can be explained and changed. The request can be placed on the agenda of an Executive Committee meeting, or the AGM. Send the Secretary an email and he'll place it on the agenda

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