A Sitesquare believed to be manufactured by Watts. Black leather case.
It has twin variable angle telescopes set at 90 degrees to each other mounted in a cast unit with a bull’s eye spirit bubble on the top. Each telescope has cross-hairs or crosslines. The base is designed to fit onto a tripod.
The device can be manually rotated through 360 degrees with a fine setting screw.
It is used for setting out right angles. It can also be used as a vertical plumb line to check the verticality of any upright line by tilting the telescope up and down and using the cross-hairs.