My 1973 450 runs fine in the morning. I drive
it to the office, about 12 miles. The car sits for 2-3 hrs, then when I start it to use it the remainder of the day it developles a very sever missfire.

Not enough information for solution to your misfire. However I would clean trigger points for fuel injection. They are located in ignition distributer below ignition points. They may be accessed by removing the two slotted head screws on the plate with the wiring harness attached to it that is on the side of the distributer. I think you will find these points, (there are two sets) saturated with oil, this is from blow by gasses that come up the distributer shaft, and condense on contact point surfaces of trigger points. This is a very common problem with EFI fuel injected Mercedes, especially the earlier models. New distributer shaft seal in 1974 if I remember correctly.

Rob