Lift up the rear seat cushions and you will see the pins that those "chrome fittings" line up with. Rear seat headrests I think were an option that came with the rear third seat option. The rear headrests act as headrest for the third rear facing seat as well.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member