Okay, is this what you have?
I'm going to take a stab at it.
On the left you have a recirc button, which closes the outside vent, and below that, a windshield defrost button, which opens the defroster vent.
Then fan speed knob.
Then the sliders allow you to open the upper (dash) and lower (floor) vents to different levels.
Then the temperature knob, then the A/C button.
Disclaimer: I have not driven a post-1985.5 944/951/968. However upper and lower vents are the only functions that aren't present on the other knobs, and it's got up and down arrows.