I agree with Atlantic great link
That is sheetrock. Old sheetrock. As you know, sheetrock is paper-faced. Paper ages and browns. Those edges are very old and aged edges of paper faced sheetrock. Yes, they are much browner than most aged sheetrock, but who knows who the manufacture was, and what was used to make the paper face. Additionally, digital cameras tend to show up staining and color variations much more so, than what the human eye perceives in actual visual presence (The cameras can exagerate the actual color difference).Atlantic,
You know what I really like about that pic? The plywood sheathing on the underside of the joists (if that is plywood).