A mystery object

Today's blip is actually an emergency blip.
When out for a walk the other day I found this mystery object.
It looks like some sort of bracket or mounting device for something. The rubber knob adjusts the position of a clamping piece, but what does it clamp onto?
There is a spirit level on the adjustment knob, and there is also dual one with two perpendicular level measurements on the main part.
I did wonder if it was some sort of photography accessory at first. Another theory, based on the levelling devices, is that it is part of some sort of surveyor's equipment - a theodolite perhaps?
It's got me wondering, so now it's your turn to give me some ideas!