I think it is part of a small plow for a garden tractor or walk behind garden tractor like David Bradley.
My guess is that this piece cuts the furrow on the opposite side of the moldboard. The square part of the blade probably slid over part of the plow's frame to keep it in place and to keep it from turning or spinning.
Do a search on ebay for David Bradley plow, then look at the pics in the listings, and you'll see what I am referring to.