mazmonza,
Just to be clear, you bought your 250 SL used and it was approximately 15 years old when you bought it, correct? So, you are not claiming that those are original to the car.
As Hans (mbzse) has already stated, "Headrests for the W113 were not available from M-B, neither as option nor standard, before Summer of 1968 ...."
So, they are not original to your '67 or CCollum's '64.
CCollum,
So, what are you going to do? Inquiring minds want to know.
If you are striving for originality, clearly they should be removed. And in your first post you stated you prefer no head rests.
But, if you like them, you should, and probably will keep them. But, if safety is your concern (you also mentioned that in your first post), you should inspect how they are mounted to insure they will truly help when needed, since they were not installed on the production floor in the factory.
Mark
(one interested in originality and safety)