Yellowing front button on 140 Pro?

[Axel]

This is what my two 220PRO's look like.

Thanks for responding.  How do you clean your 220s?

Do you use the front button to switch your amp on and off? I’m wondering whether the silver finish has been worn/eroded away to expose the brass underneath. It would be difficult for your finger to contact the small are inside th “D”, though. Is it possible it was like this from the factory and they had fitted a button that hasn’t been plated? Whichever, I like the look of the contrast in finishes.