Hilly Fields: The Mark of Cain

"What have you done? Listen! Your brother's blood cries out to me from the ground. Now you are under a curse and driven from the ground, which opened its mouth to receive your brother's blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth."
Genesis 4:10–12

This photograph was taken by local artist Leo, who approached the Brockley Society with it, who in turn have asked us to help answer his question:

This photo is taken from the path on the part of Hilly Fields that’s towards the north west (on the Montague Avenue side). It shows the ‘shadow’ in the grass showing the outline of a building or structure that obviously stood there once, and that has now been revealed by the dry weather. What was it? When was it built and when was it pulled down?

The fact that there’s a manhole cover for (?) a drain there means that it couldn’t have been too temporary. My guess is that could have been a WW2 anti-aircraft gun emplacement and/or a searchlight battery or maybe a small Victorian pavilion…but I'm open to other ideas.

We think these markings are left by the Nissen Huts, that a few BC readers have already discussed - but can anyone confirm this please?