Originally Posted by
JDNSW
It is a clinometer for measuring or setting vertical angles, but is almost certainly a gunsight - if it were a surveying alidade the rear sight would be in the middle vertically, and it would have at least one side of the base straight, probably with a parallel rule.
Best guess is that it is a simple sight for some sort of artillery piece, perhaps an howitzer. The thumbscrew attaches it to a pad on the frame, and the position of the front sight is set for the range.
John