I have these NOS rear damper inserts for the plastic car (hydraulic only not gas), Armstrong as fitted when new. Having cycled them many times and left sitting vertical the one on the right definitely has a lot less resistance in compression and rebound than the other. There are no signs of leakage so is this likely down to ageing of the seals or deteriorating of the oil?. Another thing I noticed, when fully extended it drops quite quickly and continues dropping while the other one only drops about an inch (video link). Given the difference in compression and rebound resistance I'm not going to use these, just curious as to why one should fail, same age and storage conditions as the other.
https://youtu.be/X8l91gSLdbU