You’re correct that the O-ring for the moveable piston is missing. It should be part # 350667R1. Without the O-ring there is no seal, so the check valve won’t hold pressure. The purpose of the check valve is to prevent the hitch from dropping due to pressure bleed down back in the control valve due to normal wear. It's essentially a safety valve.
I have a single acting check valve on my 340U and a double acting one on the 240U. I rebuilt both valves because the O-rings were badly deteriorated. Your photo looks just like mine did except the O-ring isn’t there. Apparently the earlier models used the single check valve.
The single check valve is supposed to prevent the raised hitch (up pressured)from dropping under load. The double check valve works in both up and down directions. In other words, what it adds is holding the hitch when it is used for down pressure, such as using the hitch to raise the rear wheels off of the ground.
Here’s the blowup of the double acting check valve. I'll see if I can find a schematic of the single valve hookup
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