Kup, do you suspect something else is happening other than sunlight damage??
The damage on the FM looks like it occurred over a very long time.
This yellowed ratchet from thetf.net that you linked us, is almost as bad as that fort max on ebay.
I've also seen the grey plastics on a dinobot turned dark/dirty yellow, whilst the clear plastic turned dark orange (almost red) in colour. However, since i'm not the original owner, i cant say for sure if it was caused by only sunlight damage or another source.
thankfully these days, no one here is that neglectful to let our prized TFs yellow to a stage where it is too hard to reverse the damages.