Our old rough cut lumber boards that was used to build barns and such had many areas like this in them. The areas where the knot holes were, where much higher in pitch content thus did not weather (rot) as much as the surrounding wood. These knot holes also made very convenient peep holes where windows were not present. I would think that what you have is the remembrance of one of these old rough cut boards. History talking back at you.
Hard to tell because I seeing it on my phone but the slot looks man made and there seems to be wear around the knot hole. I’m guessing it’s the remains of a rope tensioner. For a tent or a clothes line...
Knot sure, but if a long pole were fitted into the knot hole, the notch would work well for picking high apples. Any chance there was an orchard near there?