The air control, temp & def.  lever's are made of mild steel , zinc
plated.
The plastic knobs on the ends of them are designed to be removable to
access for instance the blower switches.
One of two things happen when removing the knobs.
You destroy the interior barbs in them, and you toss them.
If your careful, and tactful they may be reused.
They are not at all an expensive item to replace, using new knobs.
The lever's are not removable without first removing the face plate&
knobs.
Unless you bend and snap off the lever's      ( a tough thing to do )
thsy are totally reusable.

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The lever itself is made to detach behind the slotted dash plate
somewhere and to simply slide out the slot, in the event of need for
repair or replacement. Have I got this right??
What is the lever made of?
How difficult to remove and replace?
Mine are stiff and don't move easily. Without knobs they are actually
almost impossible to slide left and right because they hurt my fingers.
If my assumptions are correct, since the knobs have been removed, then
there is nothing to do but get replacement levers with knobs and install
them. Correct???