Pre-Modifide camshaft w126 SOHC Inline 6
 

I am going to replace a camshaft on a 1988 300 Se SOHC Inline 6.
When I went online to order parts it said the Following-
Camshaft NOTE: Pre-Modification cars need the following parts:
(12)103 050 10 33 Rocker
(1) 103 158 08 40 Adapter
(1) 104 990 05 22 Screw
(1) 103 052 06 01
Gear Modified Camshafts use a 7 mm (Shaft-Size) screw for the camshaft adapter.
Pre-Modification versions use an 8mm Bolt. 103 050 64 01 .

I have one bad Rocker arm and one bad lobe - #6 intake.
I am replacing the oiler tube also. The area on the tube where the screw holds it down on that end of the engine is missig metal. A chunk broke off where the screw hole is located.This may have caused improper oiling of the #6 Intake.

Are they talking about the shaft size for removing the rocker arm shafts?

For your purposes shaft size is irrelevant. You need the new parts b/c the old rotor adapter doesn't fit in the new style camshaft. It is as simple as order the new updated parts and install.