You mean the small slotted adjuster at the top, or the larger component with the retainer clip around it?
The slotted adjuster is for rebound and should match the other fork in number of lines out from full in.
The larger component surrounding the adjuster is part of the fork cap/adjust case portion of the fork cap.
Your forks are from an RC51 SP1, which use different fork caps than the SP2.
On the SP1 forks, you can count the number of lines visible on the preload adjuster as it travels up or down during adjustment.
The SP2 forks have an internal preload adjuster with an adjuster nut on the fork cap, and one has to count the number of turns to set preload as the preload adjuster stays fixed in one place.
The important take away is, you need to match the numbers of lines for both forks for preload adjustment.