Hi Daniel,
I think you are correct in the origins of the box, certainly an instrument of some sort was housed in it.
A dumpy level was used to originally survey the site but it was found quicker and accurate enough as all the tracks made are level to use a long bubble level for the track work, as we are doing now.
I do/did own a dumpy level which I gave our son when he was working on a house he bought in the Cotswolds, nearly ten years on he is still working on it!
Take care Tony.