Old or new water damage on roof interior
I don't see what looks like water damage. I see what looks like white-wash (old paint) on some of the roof 1x (one-by) components. It also looks like the white wash was put on the material before it was used to build the roof.

My guess is that the two 1x12s, with the white streaks, had the white-wash dripped on them, maybe while the other materials were being painted.

Another guess is that your pictures show some 100 year-old material re-purposing. Like a white-washed chicken coop was dismantled and the boards were used on the house.