Look - No Tension Spring!
Here are the tension discs on Maria's Jones Family CS machine. They look very similar to the tension discs on the Singer 28K that I showed in my post about thumb tabs, but there is one important difference. The Jones has no tension spring, the little wire hook sticking out to the left of the discs on the Singer.
On many old machines the tension spring has been snapped off. At first sight this seems to have happened here. However, this machine had no tension spring in the first place.
So, if you are considering buying an old machine with this type of tension discs, always check the tension spring. If it is a Singer and there is none, then it is missing. If it is not a Singer, find out by trawling through google images whether it was likely to have had one in the first place. Most other makes did have tension springs, but this Jones machine didn't.