When that car was new, you could not register a car in the U.S. with an engine that did not comply with U.S. emission regulations. I find it hard to believe, that the car had a "German engine" spec.
Although, the basic bare engine was the same, there were many additions to make the car compliant.
And yes, it is correct that the early 280SLs had a "280" only head casting mark. This is in line with the "square" combustion chamber/head gasket. It changed in 1968/69.
As for the other versions, our limited knowledge does not explain any logic behind the use.
The head on my car is original to the car and rather rare. I first thought the marking was on late cars but I found cars with higher serial numbers that had more common heads.
So far, I have encountered (heard of) only two other cars (worldwide) that had the same head. One in the U.K (SL) and one in Denmark (SEL)