I believe that some of the information posted by others is not correct.
While it is true that you don't need a smog certificate for a 1971 280SL, it is not true that you can just ship one from Europe and expect it to be able to be registered in California. Both DOT and CARB could want something. This is only complicated by the assumption that your 1971 280 SL is NOT the same as a US-spec 280 SL. I have a Euro 280 SE that went through extensive modifications before it was imported 25+ years ago. And even then it wasn't to a state as picky as California is about these things...
Things that might be different: cam shaft, smog pump, timing, injection pump settings....