There are subtle differences between the 2.6L M103 and the 3.0L M103. You need to use your VIN and engine number when ordering parts for them. Even the Transmissions are different between the two.
e.g. even though the W201 190e 2.6 and the W124 260e share the same engine there are many subtle differences in parts.
You also need to be careful when ordering parts because some specs were changed in the model update in 1988/89.

There are countries in the world where the size of the engine was restricted. Other countries charge logarithmic tax charges vs increase of engine size. M.B. does various cost options for differing engines.

I don't know why a marketing decision was made by MB to introduce the 260 engine into the U.S. market.