I'll give you my thoughts based on my '67 230SL. My lights were red with a horizonal rib. Swithing to the "newer" smooth lenses requires replacing the entire tail light unit. By the way, you might try posting this on the vintage board and get a more knowledgable response. Also if your ribed lens are in good condition, they are in demand and can be sold on eBay.

Jim Villers
1961 190SL, 1967 230SL