Oh how I hated that mirror...I knew that sooner or later someone would pick up on it. I think it was made by Vitaloni, a well know producer of aftermarket parts during the '70s - '80's and during that period it seemed to be quite a fashionable fitment. I seem to recall that something similar cropped up as standard on plastic ducktail Alfa spiders and 2nd series Alfettas and I'm glad to say that they looked just as awful on those cars as they did on mine. As you can well imagine it was the first thing that went in the bin when I got the chance. It's been a long time now but I seem to recall the original holes still being there under several layers of paint, and the extra holes for the black monstrosity were simply covered with filler - a bodge that I'm glad to say has held up remarkably well for the past 30 odd years (can it really be that long ago?) My poor 280SL currently sits accusingly, awaiting a proper restoration, and the holes will be welded properly and the last vestiges of the mirror from hell will be finally gone for good.
The only reason I can think of to explain the rash of odd mirrors on Italian SL's may be because in 1976 a new law was introduced stating that all wing mirrors had to have a homologation number on them. It was obviously a racket, just like the majority of Italian law, but a lot of people fell for it at the time, and it could well be an explanation.
Stefano