Hi John,
there is no easy answer. What is known from Engelen's book is that with chassis number 2035 the round horn ring was changed to one with a flat top and the steering wheel was reinforced to prevent vibrations. The assumption is that the wheel received grooves at that time but that is by no means proven.
What makes you think that your wheel was not changed out over the years? Similar wheels were also installed on other models of that time. It would be nice to have a picture of a documented original early 230SL wheel. The info and pictures that I posted in the topic mentioned by Rodd came from the German web site [http://www.pagodentreff.de/diskussionsforum/t2249-lenkraeder-ausfuehrungen.html] but the author provided no proofs for his statements.