IT ain't being out in the sun that causes the yellowing effect, it's just simple age that does it. And with it comes its age related brittleness! It's the old 60's plasticiser effect!!!!!!!
How do I know all this......... Handled and changed hundreds of them through yellowing and cracking/breaking due to age hardening. Not saying that this ain't the real McCoy but there's an old pommy saying that if something looks too good to be true, then it usually is!
Let's have a look at the base......