I have figured out it's a sine plate, and it seems more complex than I thought, but there are no manufacturing marks to see where it came from. All of the deck holes are drilled, and tapped, there are set screws on the hinges that are positive lock, not friction stop. One detachable rail has B&S stamped on it, and another MS is on the inside deck. The only other marks are on ground in pads with owners name in it. Under the hinge area there are screw/bolt threads visible that I have not figured out yet too. Here are some pictures, and the ad, any ideas?
Excellent working condition and very clean! Very heavy solid HSS piece.
www.ebay.com