I replaced a fuel sending unit a while back, probably the begining of the year and the unit I used was a NOS complete with BL box. It, like the one pictured, was clad in a nylon mesh which I can only desribe like a Chinese finger trap. On close inspection, there is a metalic strip, rather 2 metalic strips running tip to tip, with a slight overlap, running inside this nylon shroud.
I imagine this metalic strip opens or closes depending on the current passed through it, perhaps for a fuel light? I imagine also that the mesh prevents the debris in the tank from blocking the contact tips.
I don't know how you put a current through a metalic strip submergedin petrol, without the risk of blowing your car up. So the above is just from my experience.