The fitting on the left may have a tapered female pipe thread which would not have a proper sealing surface for the straight thread O-ring bushing.
Triptester,
I took it apart to see what I could find. I can't see that its a tapered thread, but it does have a chamfered seat which may not seal well with the copper washer. See photo:
Also, the last fitting - right on this photo, has a very small hole. Is that normal, or is this an orifice designed to reduce the speed of movement of the attached arm?