I apparently have black 901 flat tips too then. I got them from Eastmall and they say they're for the 510, but you can kinda tell they weren't made for them by the way they fit and they're glossy rather than matte like the rest of 510 products. I just thought "flat tip" and "whistle tip" were interchangeable names for the same thing, there just happen to be different versions of the flat tip/whistle tip.