Good assumption that it would be a 260E, but they made some 300Es that have the 2.6 engine. If yours were a 260 it would have the 2.6 logo on the right side of the trunk across from the 300E logo on the left! Of course, people like to remove things like that... I guess the VIN would show the original engine code.

There are a few differences between the 300E and 260E other than the engines. For example, the 260E does not come with the headlamp wipers.
I really don't know what else.

I guess some 300E buyers wanted an economy engine without stepping down on the body amenities.

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)