You áre indeed having fun Perf, it shows! lol
I think Sweper is right, isn't that simply a mechanical adjustment for the axis, with which you can center it ... Was usefull in there when there was no other proper way to calibrate the stick if I remember correctly. And if you can center the stick with it, well, you can also off-center it, ie. Trim!
There should be one for the other axis as well?
Maybe it's range is not large enough?