PDS Progressive Damping System, or no linkage, the top of the shock is bolted to the frame, and the bottom directly to the swingarm. OTTOMH, developed by Ohlins for/with Husaberg, and cloned by KTM/WP when they bought Berg and WP.
The 99 and 2000 engines were much the same and had a different ignition to earlier models too. The 2000s reverted to inverted forks, so you've most likely got a 99, possibly built in 98. The VIN will tell.
Steve