Flush Bracket holes use?

[tasmod G4NQX]

On the flush brackets on trig pillars are two holes or indents above the benchmark. Can anyone shed light on these and what they are for?
I've searched most descriptions but they are not mentioned.

[ted]

The holes are a fixture point for attaching survey apparatus; see for example
https://www.ajb-settingout.co.uk/downlo ... pdf#page=5