Brad, a leather slip-on pad that was left on the gun for an extended period could have caused that darkened area. I use this type pad extensively on my doubles to increase LOP and found that happening years go on my 687 SPII. I now only put them on long enough for use, then take them off when cleaning them after shooting.
It seems to me to be a difference in how the sunlight reacts on the wood, rather than some sort of chemical reaction between the wood and the leather itself. The reason I say that is that about 20 years ago a hunting club, to whom I leased the deer rights on a part of my land, commissioned a custom knifemaker to make several identical skinning knives. They told me about it and wanted to give me one of them. I offered a block of seasoned osage orange wood for the scales on the knives, which the maker used. He supplied the knives in handmade leather sheaths. I had mine laying for quite some time in front of a window in my shop where the sunlight shined on it several hours a day. I found that, after an extended period of time, the side of the handle exposed to the sunlight had become somewhat "bleached", while the bottom side not exposed to the sunlight had stayed much darker.
It's the only explanation I can think of.
