at first sight correct but perfectible system

the system has evolved and weaknesses encountered in the first model disappeared (difficulty tighten central nut, screw it if the system is disassembled).

by cons I can not secure the two subsets (there is too much play in setting). So the system works only with the fixed part down (but anyway that's how I found sense to use it), and above it tends to be dismantle as soon as one takes in hand.

So the better in this model but still perfectible

Change of assessment after 1 month: After two uses one of the clamps in the headrest broke. The system has become unusable. I asked for a return to the seller rather than an exchange as difficulty secure the two subsets is painful to each use and during storage phases.