So you've had this car since it was new and can verify that it's never been touched? I've never seen that on an original car but I've seen it on a 113 before.
This kind of reminds me of the car with factory hood insulation. It's factory original done by a MB dealer in most cases.
It might be on your original, never touched car but it's not believed to be original. I'm more inclided to believe things are original that haven't been added rather than something that's not normally there and somehow was put there.
I saw a 100% original and very low mileage 230SL last year and the grill area was NOT blacked out. The blacked out areas were hand applied with a paint brush so it's totally believable that some cars didn't have this treatment. Normally they do but this one definately didn't.
So, it is possible yours is correct but all the info I have points in another direction. Be interesting to find out for sure. I do think that most of these oddball things are more dealer than factory in almost every case.