The pressure plate on the left is called a 'spring' type, as typically found on early 850cc engines. The one on the right is called a 'diaphragm' type. While the pressure plates are NOT interchangeable on the flywheel itself, either flywheel can be used on any engine assuming you keep the flywheel with the correct pressure plate. The back plates and the clutch disc itself are interchangeable.
Pre-Verto versus Verto Clutch
In 1980, the Mini’s clutch underwent a major change. The internal clutch/ flywheel assembly changed as well as almost all of the external, connecting bits as far back as the master cylinder. They are pretty easy to tell apart with just a quick look inside the engine compartment. Here are two ways. There are others.
Clutch arm. The clutch arm for the Verto set up is a short, stubby little thing. The exposed length is only about 2” while the pre-Verto clutch arm is closer to 6”. With such a wide difference, there is no need to measure.
Clutch slave cylinder mounting. The pre-Verto clutch slave is mounted horizontally directly onto the flywheel housing about level with the top of the long clutch arm. With the Verto, the clutch slave cylinder is mounted on a bracket which is mounted on the flywheel housing allowing the clutch slave to point downwards at an angle to meet up with the clutch arm rod coming off the end of the very short clutch arm. The slave cylinders are visibly different, too.