Lot's of suggestions floating around. I agreed at first that it may have been a mower sickle bar guide, but after searching for them, all I could find were the 2 hole type, which have a similar shape. I'm not convinced it's a planer cap, either. It looks too "roughly cast" plus with hundreds of examples online, none matched . With the great variety of farm implements, I'm still leaning towards some type of guide, or hold down clip or clamp, specifically designed for one job. That's the problem with ID'ing parts. Many were created for one particular machine, or even model.