http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD2/Program/Chassis/72-185.pdf
There are normally three sliders.
The "extra slider" is the front "slide shoe" that attaches to the front lower regulator arm (Fig. 172-29717 by step 6).
It may have been left behind from a prior repair of the regulator arm, and that's probably why you have an extra.
The metal pin that attaches it to the regulator arm is missing.
The rear slide shoe attaches to the rear lower regulator arm (right side of Fig 172-29710 by step 7).
The front and rear slide shoes are attached to your regulator arms, so you can safely remove and discard the "extra slider".
The "mystery slider" is partly visible in the left side of Fig. 172-29710 by step 7.
It ties the rear vertical guide rail to the horizontal guide rail.
It should be re-installed.
The three sliders are also visible in the photo on Post 5 of this thread:
sticky or DIY for front window regulator replacement? - Mercedes-Benz Forum
Hope this helps.
