I need to ask one question of you. Are the seats/leather or MB-TEX original? If they are not and have been re-upholstered, the possibility of leaving headrest off seats for aesthethics/looks could have happened and the leather/MB-TEX was not cut with holes for headrest post. I do know that new leathers for seats come with no holes cut. You must cut them. This is so that the seatcovers will be compatable to all models in the W113 series and will fit correctly w/holes at proper dimensions for post. You may remove the seat backs and see if the housings for headrest rods are there. They screw to the back frame.
Bob
Robert Hyatt
Williamsburg, VA.
1970 280SL, Red on Silver,4-speed, Euro spec; 1999 C230 Kompresser Sport, Black on Black; 1990 Miata, Black on Red, 5 Speed