This was a piece that was sitting in the fields for who knows how long. I did manage to get it going yesterday. Needs a needle bearing on the crank arm, a belt, and could use all new teeth, but it works! I guess it's not critical to know the manufacturer, but it would sure be nice.
As far as I can tell, the nearest to it seems to be the Ford sickle bar mowers. They have a very similar tube frame, and some other similarities with the belt drive. But the 3-point setup on this one I haven't seen anywhere else. Especially the top link.