For what's worth ...
fully agree with Alfred's and Hans' findings.
Head rests were an option from VIN 004052 onwards for the 280 SL non-US versions. For US-280ies standard since then.
By the way.... the seats were not different for head rests or non-head rests as well as there are
no differences between seats for the US and non-US versions!
The Pagoda seats changed their design several times between 1964 and 1970 with most changes taking place during the production run of the 230 SL, in particular within the built (= calendar) years 1964 - 1966 (i.e. seat types 1 - 4).
This includes the fact that the late 230 SL seats (from VIN 017861 onwards, seat type 4) had the thicker backrest in exactly the same way as the 250 (seat type 4) and 280 SLs (seat type 5 = different seat covers).
And that's why Fritz68's seats are incorrect for a 280 SL (seat type 2 = 230 SL between 002927 - 012466), sorry to say so but I stand with what I mentioned before.
Head rests were available only for the 280 SL (from serial 004052 onwards, as indicated above) but can be retrofitted to any of the earlier cars as well. See pictures attached.
IMHO
Achim
(ear-headrest collector)