Note arrow A, on the front assembly, how the retaining ring gets completely captured by the threaded adapter against the ridge formed in the body of the damper.  However, I could not get the ring to fit under the threaded adaptor of the rear assembly (B).  That adapter was aluminum, but because of how thin the wall was made, a steel reinforement collar had to be added to it.  I eventually sanded the outer diameter of each ring down a little to get them to fit.  This should be OK, since the part that was sanded off was the smallest part of the ring.  Thinking about this afterwards, I should have paid more for the aluminum bodied dampers.  Their entire bodies were threaded, so they required no adapters on the body, and they had external damping adjustments.