Barry this what they refer to when off road towing is discussed,
Off-Road Couplings (Non-50mm ball type)
Off-Road couplings are designed for use where high degrees of articulation are required. Some use a separate pin to connect, whilst others use a built in locking mechanism. Many have polyurethane components to absorb shocks.
All of these couplings are required to incorporate a positive locking mechanism plus a separate means of retaining this mechanism in the locked position. This locking must be readily verifiable by visual examination.
The use of tools to engage or disengage is not allowed on couplings up to 3500kg.
Both parts of the coupling must be marked with the Manufacturers name or trademark, the words “use with model (identified model)” and the maximum allowable trailer ATM i.e. 3500kg at which the coupling is rated.

