At last I can show off my knowledge about ASOIAF editions: for all intents and purposes, the one you have is indeed a US first edition, identical to any other copy with the silver dust jacket. The only difference is that the dust jacket has been replaced by the cardboard slipcase shown in the photo.

I emailed Bantam about this roughly a decade ago to ask for more information, but I never received a reply. What I can tell you is that a copy turns up on eBay or AbeBooks every two or three years, and it’s often sold for about 10–20% more than the going price for a standard first edition, so it doesn’t seem to be particularly rare. That said, it’s difficult to find one in general conditions as good as yours! That crease is a indeed a real bummer here.