Surveyor's tool?

[Mitica100]

A friend of mine purchased some old surveying equipment and ended up with this little piece:

It's made of polished oak, brass fittings, has a mirror inside which can be adjusted laterally, the arm swings in a 90 degree position enabling the little piece of lined paper with a hole in it to line up with the mirror. It has a little circular knob (brass) which seems to be doing nothing (broken?) and has a hole at the bottom where you can stick it onto a pole, I think.

I have no idea what it is (obviously he doesn't either) so I'm throwing this at you, guys and gals of the TPF hoping someone might know what it is.

Thanks ya'll...


PS I have more pictures if someone needs more info.

 

[Artemis]

Check if a walls straight??