I'm assuming that the handle with the knob on is an over-cam tightening device that locks the plate down? and if you turn the other handle the arm lifts and then drops suddenly? or, would it stretch something attached to the plate and fixed over the pin?
Have you tried a google image search? Prob need to take pics with a plain sheet on the floor and as a backdrop to cancel out all the other stuff.
What is the history of it? Did it come with the house or have you just found it? What do the handles do? What moves and what doesnt move? Is that a lead weight or stone in the upright moveable piece?
I would go back to the source and ask them. Local museum, library ect for knowledge. Take some pics and put a notice on the supermarket free addds notice board. Are antiques roadshow due in your area soon? Might be worth a try, local antique shops.