On the W108 and W109 sedans the water valves are directly behind the ash tray. Remove the ash tray to view the water valves. You can also remove the glove box to gain more access.
Heater lever replacement with originals is a bit more difficult on sedans than the W113 cars. There is a hidden phillips screw on two lower controls which locks two control cables. The screws are only visilble by viewing from the outside of the car, through the windscreen and into the speaker opening (speaker removed). And then is only visible when the lever is moved to a certain position. You will see it emerge in an opening in the casting. At this point the screw can be loosened and the cable removed. In addition the top left lever on sedans is more complicated than W113 since, on sedans the heater fan switch is built into the control lever itself.
The "Durolevers" that you have, were a fix to avoid all the work of replacing the originals, but you will still need to free up your heater valves.
To free up your heater valves, drain the radiator add radiator/engine flush and water. Run engine until full engine temperature 180 f , then attempt to move the valves. The heat and flush chemicals will usually free up the water valves. You might have to let it work for a while. In the end they should move smoother than ever. Don't forget to drain the flush out and add new coolant when finished.