This is a letterbox, please don't take the stamp
Start you trek at the posted coordinates. From there you will see a retaining wall. Go to the middle of the wall where it is tallest and you can reach over the wall and feel for some fist size rocks. The letterbox is nestled under one of the wall rocks that form the ledge.
Unless you are short you should be able to reach and feel the cache from the sidewalk side of the wall.