I asked for C4 and they sent C3. Unfortunately, other buyers mention this in their comments as well and they were right. They send whatever they find

Within 3 months it went off and it starts lowering by itself with daily office use of 8 hours and normal weight (80kg)

It fits perfectly to the standard dimensions. Very strong return. It was not greased externally, resulting in squeaking. Grease was applied and the squeaking stopped.

Attention to those who order the class-4 that supports up to 150 kg. They send the class-3. That's how I got tricked too, I sent it back, ordered from another supplier, again class-4 and they sent me the 3 again. I had to keep it because I needed the chair even though I lost 3-4 euros.

Very easy installation. The difficult part is removing the previous shock absorber, for which you need a good hammer and quite a bit of hitting in the right spots. It seems sturdy and works perfectly so far; time will tell about its long-term durability.

So far it works properly. It was installed easily (removing the old one was difficult but that's to be expected). The dimensions matched. Definitely satisfied. As for whether it meets the specifications or how long it will last, I won't bother further, we'll see. In any case, it doesn't cause any reason for concern.

I don't know, but for me, even though I set it all the way up, the height of the chair slowly drops; every day I have to readjust it.

It's a simple shock absorber, it does its job. For 10 euros, I can't complain. You remove the old one with a sledgehammer and install the new one. It can handle 100 kilograms and more.