Historical - Angle iron used for Survey Bench Marks

On one of our recent jobs, we came across an old benchmark, I was wondering how surveyors used their angle iron to support their levelling staff / rod, the only picture I could find of one of these angle irons, but seems incomplete, there must have been something to prop it against a rock, does anyone have photographs of an angle iron in use?

https://collection.sciencemuseumgroup.org.uk/objects/co52873/angle-iron-to-support-levelling-staff-at-bench-mar-bracket-structural-element-bench-mark