I have a yellowed tank (not L-plug, but straight) that is of unknown condition, i.e. presumed much needed TLC. But I can test it to see how it works and let you know; take some pics, etc. if you like.
I believe it to be pretty early days, as it actually has an AMIGA logo on the bottom - which I believe to be rather rare. None of my other tanks have that for some reason, not even my A1000 tanks.
It would need a retr0bright or some other type of refurbishment to look good, I know that. And I'd be willing to bet the internals could use a big clean up and brushing up, too.