Here is a screen shot from the technical manual that shows the different in the radio size openings. The early cars have a 230 SL and 250 SL have a separate cover piece in the dash for the radio. The later cars have a one piece dash.
PS - also I will chime in to say that you should consider a full membership which gives you access to entire technical manual, which is a hgely valuable resource, along with the Pagoda World magazine twice a year, and four electronic issues of Pagoda Notes newsletter.
