In 1987 Porsche made five 928 S4 cars in the specifications of the CS.
These five cars were build for the factory drivers: Jochen Mass, Hans Stuck, Jacky Ickx, Derek Bell and Bob Wollek. 
These five cars were most similar, apart from Stuck’s PDK gearbox and Bell’s LHD car but with a speedometer in miles. With this in mind and remembering that these cars were lightened one can legitimately say that they are the 5 prototype 928’s for the CS model that was introduced in 1988.
As Derek Bell was reluctant to take delivery of the car because it was LHD and in fact initially refused it, it remained for a while a Porsche AG car. Therefore the first owner of the car is Porsche AG.
Approximately a year later he asked about a white 928 sitting under a tree to be told “oh that’s your 928”. He then accepted the car and brought it back and registered the car in the UK. It had covered approx 20,000 miles – although Derek had a new “mph” speedo installed.
Derek knew it was one of 5 unique 928’s built. All 5 were given to Porsche racing drivers. All were white and LHD. The car is a 1987 model year prototype S4. This is identified by the VIN number and the folding rear spoiler (which was only fitted to 1987 model year S4’s). He was also informed the car had uprated suspension and engine. The car was also lightened.
Derek was until recently the only driver of the 5 to have kept his 928. He kept it with his other ultra rare Porsche – a red 924 Carrera GTS. He has covered approx 42,000 miles in the car before selling it.
Without a doubt it is the rarest and most unique 928. 
Adding to the fact that this 928 is already a rare car, it is a low mileage car first owned by Porsche AG, then by one of its most successful racing drivers Derek Bell who sold it to the previous owner. He was the third keeper of the car and only drove it occasionally. It is now residing in a collection together with some other rare and special Porsche’s.