I think matte proof is your only real option? It's 90%, and an S so it's a proof. The mirroring on the background is consistently absent. On standard proofs the mirroring doesn't consistently wear, so I'm guessing matte is exactly what you have. Very cool.
Can you show me a photo of a 99S matte proof? I'd like to see one. Since this coin has been in circulation, I suspect it's had a ride in a pants pocket through the washer and/or dryer. That would give it a nice even "scuff" over the whole coin.
Does it weigh more than 11.3g? Personally I think it might be a silver dipped copper nickel half dollar based off of the picture of the edge you provided. I'm not an expert so please don't go off of my opinion though, that's just my guess.