I looked up some info as I was curious myself.......
Year but also...Trim level.....some have chrome trim on top, some on bottom, some have just the side markers with nothing else or the trim is painted like this one you show. Most of the "trim", like on my 78 1:1 Mustang II, was added at the dealer. But on my 1:1 the guy said he added the trim over the wheel wells.
There is also a vinyl strip trim (to match a vinyl roof) that was as thick as the markers and ran level in between them.
On the model kit I used the trim is on the bottom.....reissue AMT 77, I looked on a few old MPC kits and the trim line is level in the middle with the markers...and the MPC 1976 Mach 1 has no trim lines, just markers..!
Good eye......I never saw that.......