Early W123 climate control help

I finally have the climate control on my 1977 300D pretty much sorted
out with one exception, the defrost flaps stay open all the time. Does
anyone know where the actuator is? I have checked both actuators that
are visible under the left side of the dash, and both appear to hold
vacuum well. Ditto for the right side. The manual says that it is a
dual diaphragm actuator, but all the ones is see are single diaphragm.
Do I need to remove the dash to get to this?

TIA

Michael.