No. Low on refrigerant will show a difference between center and outside vent temperature sometimes. A side to side difference split at the middle of the center vent is duovalve. Guaranteed.
Later or upper end models had the dual zone climate control, and on the earlier models, the system was the same, but the controls were not implemented. The duovalve can accomodate dual zone climate control, as the heater core has 2 separate circuits, each controlled by half the duovalve. On cars without dual zone controls, both solenoids of the duovalve are controlled by the same signal. When one solenoid fails, the difference in heating is split dead center in the middle vent. This failure can also be distinguished from a flap failure by the fact that the side vent is involved as well.

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'96 C280 (gone)
'97 C36
'05 C230k