I saw this springboard notch in a stump on the side of the trail, and it reminded me of a mail slot!
This is placed on the well maintained 1.5mi Powerline Trail. It starts at the beginning of the WTT trailhead, and, after crossing the WTT twice, ends near the Long Lake Overlook. It is a much more pleasant alternate then the steep slog up the WTT.
The trail comes out at the WTT at (N48 43.831 W122 18.196), and then again at (N48 43.747 W122 17.989). In both of those cases, it just starts back up on the other side. The trail ends at (N48 43.524 W122 17.655).
Happy Hunting!