I can tell the difference between MB tex and leather by the smell. When I get in the 77 300d on a warm day, I can smell the leather. No smell with the mbtex in the 85 300td. You can also look at the material and see the difference. I'll probably find a piece of raw leather and put it under the seat in the td to create that pleasing smell.
Third seat was an option. Mine does not have one. You can buy third seats on ebay. Most include the seat belts.
I know the back seats had headrests. Mine are missing also. I assume it's because you have to take them off to put the seats down and eventually someone got tired of taking them off and on. If you find a source for them, let me know.
Some td's also did not have the split (two-thirds/one-third) back seat.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13