I knew it was Smythe's picture... it's written right on it.
However, the drawing shows the last slot to be a bit of a distance up from the northern wall of the gallery. Petrie's picture shows it to be right against the northern wall of the gallery. My memory says it is against the northern wall, but it is covered in a wooden platform now, so it is hard to be sure.
Here's the picture again, in its webpage setting: [
www.ronaldbirdsall.com]
One of these illustrations is wrong. I do think it is Smythe's, which is the one you posted.
Anthony
You can lead a horse to water but you can't make him think.