What LEGO piece is this arc with ball joint? I cannot find a part number on it. It has the official logos on the studs. What is this? What theme does it originate from?
It looks like a combination of a 4x4 plate, a 5x2 plate and an arc with a ball joint.
That is the Vehicle, Trailer Base 4 x 13. It came in one set, the #378 Tractor from 1972.
Found by searching first 4x9 (didn't find it) and then 4x13 (the total length).