The rigid ones were used then. You would use a big wrench, like a Cresent wrench, to grab onto the idler wheel holder. Then you would loosen the bolt holding the idler assembly. This bolt is behind the mounting bracket.

Then you would adjust the tension on the belt by moving the entire assembly up or down and when you had it where you wanted it you hold it in place and tighten the bolt.

This is a very simple system but it is easy to over tension the belt and cause undue wear on your idler pulley bearing. So tight is good on the belt. Piano wire tight is a bit too much.

When you see it in person it is easy to figure out.

By the way, the idle pulley is a special one that Mercedes uses since it has to fit the bolt that goes through the middle of the idle pulley. This bolt has a taper to it. So don't try to pick a pulley up at a hardware store and expect it to fit.

The good news the pulleys are very high quality so when you install a new one you are usually good for the next ten years.