Thanks to everyone who replied to this post before.
Now I post my replacement notes.

After some research on the new gas-operated tailgate supports, it turned out that all I could find are made at the same place! Though I had two different ones on my wagon, one marked VOLVO, another Lift-O-Mat, fine print says it was made at 56070 Koblenz, Germany.

The brand new replacement supports I got yesterday for approx. $11 each are also made at that location and quote their web site, www.stabilus.com
This really nice site shows the construction of the shocks and gives some tips,
such as about proper installation: "Whenever possible, gas springs should be installed so that the piston rod points down in the inactive state, as non-orientation-specific installation requires special consideration. This ensures optimum lubrication of the guidance and sealing system".

Now I am happy about the new supports! If with old ones, I had to move my tailgate all the way up by hands, and then check if it is not going to fall down (particularly in colder weather), the new supports would just lift the tailgate themselves after you unlock it and move up maybe just a couple of inches.

Before installation, I also measured the extension force of a new support. It is 30kg, or 66 lbs.

/Wolandir