Hello Eggplant,
Too difficult to answer. The parts number for the ashtray, 1138100030, never changed throughout the production run.
However, there are many little details which changed over the years.
And yes, there was a more "flat style" of the lid. I have observed that one only on original 1964 cars. Later those seemed to be changed into the shape that is still available today.
Other changes are the spring for the lid, the addition of the GHE manufacturer imprint ... The grain of the padding clearly changed several times over the years.
If you open the lid and look at its inner surface, some have flat chrome (i.e. brushed) others are polished.
Recently I found and old (and rusty) ashtray without lid which has clearly different openings (for the ashes), these are smaller!
I am still doing research on this...
For your case (250 SL) I would go with what you can find today, don't become crazy on these little differences.
If you want to do it exactly you have to look at the real survivors like John's ("The Lews") red 250 or the light blue late 250 firstownercar of "Van White". The latter was posted a couple of years ago, I believe it was still on the Yahoo site. Most pics of that car are from the completely authentic engine bay. I have to look whether interior pics were posted too but I doubt.
I recommend: John (Lewenauer)'s.
Best,
Achim
(no SL but parts & ashtray collector)