Well, I'll try to describe it as good as I can.
On the 550 forks, the piston sits inside the chrome tube. One way to lower the suspension, is to put a tube/big washer between the piston head and the small spring (BLUE ARROW), which makes the piston sit further in in the tube.
On the 500 forks, the piston looks totally different.
The piston doesn't sit in the chrome tube, which makes the idea of a tube washer an unfitting technique.
The only way I can think of lowering the suspension with this design, is to:
* Shorten the rod at the top and make new threads,
* Cut the spring the same length as the rod is shortened
* Cut the chrome tube at the bottom and lathe the inside a little bit (originally it is, but by shortening the tube the lathed bottom end will be cut off), so that the bottom end of the piston fits inside (which it otherwise won't).
Is this really the only way to shorten this kind of suspension?
Would be easier just to buy new forks, but those kind of forks aren't available everywhere here in Sweden..