AFAIK the 4 rubber pads under the car are where you are supposed to lift it from. Thats where the lifts seem to grab the cars. The holes right above those pads are for the tire jack. In front the W126 has a nice cross member right in front of the pad, if you use a block of wood under the jack you won't even break the under coating. I will never go under a car unless it is supported be a very good set of jack stands with chocks under the wheels that are still on the ground.
For like $35 I got a real nice set of Chraftsman jack stands. They are rated for like 8k pounds each.