Good question!

I got out the window sticker for my '88 300E. In the standard equipment list is: "AERODYNAMIC HALOGEN HEADLAMPS WITH WIPERS, AND WASHERS."

This car came with only two options; 904 paint and 274 Palomino Leather upholstery $1,425.00 and option 412 Electric sliding roof with Pop-up N/C (no charge).

The only answer I can imagine is that the earlier models had them as an option or not at all. Through most model life cycles, there are options on the earlier models that are thrown in free with later models of the series to maintain interest.

Good luck,