The Cranes of Ibycus

Travelled up to Birmingham today for an ALT strategy meeting. I was on the train at Cardiff when it was announced that all trains were cancelled. By coincidence my colleague Simon Horrocks was also on the train, heading to Milton Keynes. He drove us to Bristol.
Here are the cranes on the Cardiff station redevelopment. In the story of Ibycus the killer gives himself away by pointing to the cranes. I'm not sure what these ones mean