This seems to be an old version of pipette. After tinkering with it for a bit i found a way to adjust it, but still not perfect. Sharing my findings for the benefit of others. This is an older version with SN: L16004J.
This version does not have screws holding the adjustment wheel, but rather the top of the pipette, the back of the wheel has a nut that unscrews. The depth to which this nut is set controls the travel of the plunger. Theoretically it should be an offset adjustment, not slope. There are no set screws holding the nut in place, so turning it you adjusting it.
After tinkering with position of the nut i determined that any position other than real tight, which is how it was, makes things worse. The thinned down portion of the plunger goes through the center of that nut and that nut determines plungers travel distance. The piston, which is what actually doing the fluid evacuation is moved by this plunger. I added a 3mm washer to the back of the nut to push plunger forward. This addition has made things better but i am still dealing with slope error. Before adjustment i was 22ul over at 1ml and 40ul over at 100ul. I am now 15ul under at 1ml and 10ul over at 100ul.
Hope this helps to anyone else that is trying to calibrate Gilson P-1000