It looks like a complete set of parts. I think the top mechanism is upside down in the picture, if that helps.
If the spring really is too short for the hole in the pitman arm you have, it would seem that putting a plug or some washers in the hole would help the spring apply pressure. The plug might need to have a hole drilled down the center to accommodate the shaft.
At any rate, the mechanism should squish down pretty tightly onto the ball head-end of the knife. I keep a screwdriver handy to push into that little hole on the side of the clamp mechanism, which then provides leverage to open and close the clamp. To fully close the clamp on mine I need to first push it most of the way with the screwdriver, then step on the flat tongue that sticks out toward the knife. You shouldn't be able to open it with your fingers.
Good luck, and happy mowing!