Sorry, but your suggestions are based on wrecks of one SB-2 only. Some painting are looking as degraded or heavy-weathered silver paint.
From wich part of the airframe are on last two pictures?
All photos from one plane.
I know what you talking about
You might convince me, if I had based my opinion only by a photo, but I saw it with own eyes, in a beautiful, clean and undamaged condition, and see how look degraded silver too. Silver paint never look at it. Let me remind you that the SB painted silver is extreme rarity, mostly these "silver" SB was not painted at all, as such for example:
This is how look degraded silver: