those are the jet intakes on an S series, they went so much faster than R or L series they had to add intakes
actually they are trim to make sense of the new style hood on the old style cab, older L, & R hoods had a bulge along bottom that matched body contour, S series eliminated that bulge. they all share the same cab,,,more or less.
bad news is they are hard to find, good news is they are the same on larger trucks, so if someone is parting a big ol farm truck somewhere they will fit your pickum up.