Maybe the stuff that removes yellowing on car headlamps?

That looks like fade to me.

I would have recommended Novus 2. If that doesn't cut what is on there it is either fade or embedded in the plastic. Either way if you haven't gotten it out now I doubt it will come out.

Mike O.

Reincarnation? Seriously, I dont know anything that will get the yellowing out of plastic once it's like that. Like Mike said if Novus doesnt get it out usually its perminent.

It may be better than nothing to someone who's game doesn't have one.

Mike O.

If it's yellow coloration on the outside of the plastic you might be able to fix it like with yellowed headlights. Toothpaste on a buffer wheel works well. Colgate is my preferred buffing compound but any toothpaste with some small amount of grit would work.

I'm sure they have actual buffing products to do the same thing but toothpaste seems to do the trick.

There's a good chance someone will want your old one. Apparently remaking speaker panels isn't common. I'll bet someone out there has the wrong one in their machine and would be happy to take this one.

I used the Scrubbing Bubbles bathroom cleaner on some nicotine-stained plastics, and it cleaned them quickly with no damage.

There's a method used to un-yellow old computer (ABS) plastics that uses hyrdrogen peroxide and UV light. No idea if it would work on pinball plastics, but I've always wanted to try it. At this point it might be worth a shot and you could report back on the results (although it is a slightly involved process).

http://retr0bright.wikispaces.com/