There are two ways to be able to differentiate the years between the 1979-1982 LTD 'S'. First of all, the 1979-1980 had a slotted front bumper like this one:
http://www.imcdb.org/images/046/448.jpg

the 1981-1982 LTD's had a solid front bumper like this one:
http://www.imcdb.org/images/026/061.jpg

Also, the 1982 was the first and only year the LTD 'S' had the blue ford oval on the grill:
http://www.imcdb.org/images/108/969.jpg

I assume the bumpers apply to the four-eyed LTD too?

I don't know about the 1983-1987 bumpers, I think they were all the same. The only way to tell the '83-'87 LTDs apart is that 1986 was the first year they had a 3rd brake light. I'll go ahead and comment the corrections on the site, since some people (particularly Snoo...well, I'd rather not mention names , insist on making guesses for the particular years.)

I'm just saying '79-'83 had four-eyed LTDs too and wondered if the bumpers were the same as the LTD-S.

Oh, sorry, got that confused. I'll try to check into that and post whatever i find.

qwerty_86, I think that the change in bumpers also applied to the four-eyed LTDs.
This is at least a 1979:
http://www.imcdb.org/images/002/879.jpg
And then notice this 1982:
http://www.imcdb.org/images/079/937.jpg