Spares or repairs panels
 

Ah, remember when there were ashtrays for the rear passengers? The best ones were on my car :( I haven't cleaned these, obvs, but vinyl cleans up well. Tip: for tough spots those wavy sponges with embedded micro-shards of melamine work really well.
Another tip: if you take a door panel off a 240 you will notice a thick sheet of polythene taped over the whole panel area, which stops the hardboard getting like these in the first place.