I've seen 1980's thru 1985 gas & diesel Wagons with & without center seat Headrests & provisions. All my Euro Wagons came with them. It doesn't list as a specific option for my cars.
So I wondering what determines what Wagons have Headrests & don't.
The ones that don't have Headrests, don't even have provisions to install.

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83 280TE- 5 spd, 16.5mm rear sway
85 280SL- 15" Bundts, 5 spd

It's my understanding that the middle-seat headrests only came on cars with factory third-row seating, to prevent backs of heads from banging together, one would presume.

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Robert Fini

'12 ML350 BlueTec, 87k
'06 E320CDI, 270k
'05 T1N Sprinter 2500/Pleasure-Way Plateau TS, 69k
'97 C36AMG, 313k (son's)
'94 E320T, 249k
'93 190E Sportline LE, 168k (daughter's)
'84 190E-2.3/5spd (Stage Rally Racer)
'66 230 W110 Sedan (Barn Find, Vintage Racer build in progress)

That may be the best reasoning. I do own a 1984 280TE wagon w/o 3rd seat that has the center seat Headrests & use to own another 1980 280TE with same arrangement

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83 280TE- 5 spd, 16.5mm rear sway
85 280SL- 15" Bundts, 5 spd