What´s that thing on the pickguard?
Left Handed-Thumb rest ?
Or is it for people who play with their thumbs?
It is a thumb rest but it's beyond me how it would work
Hook your fingers onto it and use your thumb on the strings.
Actually, it is a finger rest. The idea was you grab it with your index and middle finger and play the strings with your thumb. When thumb playing gave way to finger playing, they moved it to the top of the pickguard and you rested your thumb on it while you played the strings with your fingers. So many people took them off that Fender just gave up on them altogether. They put them on reissue models just for the sake of making the basses look like the originals.
Watch old 50s and 60s footage, and it will make perfect sense. Most of the pop guys that played Ed Sullivan, for instance, used the finger rest and plucked out notes with their thumb.
yes, there are previous threads on this subject, it's called a "tug bar" and you use it by holding it with your fingers while you pluck the strings with your thumb.