It’s a common misconception that the ’79 LTDs with the single headlamp on each side are LTD S models. This is not the case. Those were just the base LTD that year.
When the base LTD received the quad headlamp treatment for 1980, Ford kept the old arrangement around for a budget model called LTD S.
The LTD S did not officially unveil until the 1980 model year. The S, for its first year, was available in 4-door sedan and station wagon styles. No coupe was offered. Engines were either the 302 CID 2bbl V8 or the 351 CID 2bbl V8.
For 1981, the LTD S was marketed only to fleet customers such as rental, taxi, police, etc. Because of this fact, it was rare to see one in any brochure or ad starting this year. A new engine arrived this year which was a 255 CID 2bbl V8. the 302 and 351 were still around.