Be patient these manuals do show up on ebay, which is where I found mine.
I will make a distinction on the 68 250SL question you asked, by saying that the real difference came in the middle of the model year, with serial 2980. This is when the safety equipment for the US came into place for the 250, different interior parts and pieces, door handles, door pockets, various safety related changes, (you can read thorugh Meredith's book, and see a bit more detailed list)steering wheel, heater levers etc.
So if you have an early car, yes the manual will be different. I have not made a side by side comparison to know if the mechanical items would be substantively different, things like maintenance routines, grease points, tuning, capacities. etc.
Jonny B
1967 250SL Auto