What is interesting John, is that two sources of information "we" usually go to are wrong on the subject.
There is a lot of reference to works by the late Frank Mallory--not the Frank Mallory who posts today (that's his son I believe). Frank states in a comprehensive chronology of production changes for the W113 that headrests appeared on US cars at start of production in 1968. We know that to be false. My guess is that was a typo that was never corrected.
Then there is the [in]famous Meredith book on SLs, and that has a comprehensive list of changes (appendix in the back), and specifically mentions a lot of DOT and safety related additions to US cars. I've read that over and over and unless I've missed it, I can't find any reference to headrests.
If you dig deep and research you'll see that an astounding number of people suffer whiplash neck injuries each year. Even with headrests in our cars, if your head isn't resting on the headrest when driving (or pretty close to it) it probably isn't worth a darn. I'm forever lifting mine up and tilting forward to make it somewhat useful.