There are 1 or 3 side buttons? I only found tech.specs but no manual.
... but there was one another issue. I wanted to use Fujitsu GL5600 without drivers but it has unstandartly connected side buttons: the front one is for BACKWARD function and the rear one is for FORWARD function. I solved it by disassembling of mice and change PINs of buttons simply (NO SOLDERING).
Now this is the best mice in the market for me.
It have 3 buttons. The middle one is controllable after installation of drivers. Manuals are here http://support.ts.fujitsu.com/com/support/manuals.html