In the end the joke was on me. I tried to pick up this hammer wondering whether it was left over from some construction project. Nah! This again, including the benches in the background, is at the Sodo Station in the industrial part of town. There are tools on the benches too. They don't move. They are art. Got me!
If you look carefully in the gateway photo, you'll see an arc "scribed" into the pavers by the pencil, much as you'd do to draw a circle with a compass. It all makes my little mason's heart flutter.
Keep it plumb and square,