The left hand side of your fork has a Air Preload input.
The 2004 Z150 had a Rebound Adjuster and "No" Air Preload input.
It is not possible to attach a HSCV Cartridge to a marzocchi Air Preload cap, so the fork does not have the HSCV cartridge on that side, That Cartridge is what makes a Z150 a Awesome high end fork in it's day. Without it it's just a budget Marzocchi fork. (although the ETA is nice)
When they don't use a HSCV (high speed compression valve) Cartridge, instead a simple plunger arrangement is used (called SSV, or SSVF), some had Adjustable rebound in that year, but the adjuster is at the bottom of the fork (such as Dirt Jumper 1, or Drop off 1), or internal using a long allen key from above (such as Dirt jumper 2 or Drop off 2) (adjustable using a 10-12" long allen key usually 3-4mm size) Often the simple non adjustable plunger was used in cheaper forks, like dirt jumper 3 or Drop off 3/4 models.
Juls