Cigar smoke 
That's what I'm thinking, but then wouldn't the keycaps be yellowed too?
Model M's case isn't PBT as far as I know though, so maybe cigar smoke yellows it anyway.
I know the Greenock made ones had ABS spacebars with that weird grounding thing but I don't think they did it with the whole board.
You know, if it was smoke, wouldn't it be uneven? And surely there'd be a HELL of a lot of smoke to yellow the top evenly and fade away - was it inside a 3ft cube room? Also, the parts not typically exposed to sunlight - the bottom and the rear lower-half of the bottom case is beige with a strage gradient on the back.
I'd expect to see burn marks on it too - I did pick up a keyboard from a smoker once and there were remnants of ash marks and browny/orange lines from where they'd placed the cigarette on it. Cleans up fairly nicely though.
Also 2.3KG heavy - is that typical?