The MK3S i3 has a bolt horizontally through the extruder stepper below the inspection opening for the gears. It turns and the nut also does as it is not tightened like a normal bolt. This seems to be an adjustment for something. What does it adjust and if turned in a particular direction what does it do?
That is the filament tension adjustment. There is a string and you can wiggle the door (with the tensioner attached)
step 27 and 32 are most relevant.