All correct. Being an original Chamonix car the seats are trimmed in dark blue cloth S 48/39. Hard to believe but the upholstery is immaculate. I am not sure, however, if the seats are the original ones. As a USA model the car might have started with dark blue vinyl K 49/39. In the USA, to my knowledge front headrests were required from model year 1969 on. The New Horizon should have had them when new. They probably got lost at some point in life which didn't matter because the car never went to America, anyway, see above.
Headrests were optional for the 2500 most of the time for most countries. In Sweden they were mandatory from the beginning, I guess. For Gemany, it was as follows:
Series 1 (68/aut-71/spr)
front only, 2500 optional, 2800 standard
Series 2 (71/spr-73/sum)
front & rear, 2500 & 2800 optional, 3.0 S & 3.0 Si standard
Series 3 (73/aut-77/feb)
front all models standard, rear 2500 optional, all other standard