Pay no attention to the naysayers CC. If you already have a double flue tiled chimney you are good to go whether or not you you find a thimble already in place (as I mentioned before). Forget about the stainless liner because in actuality, tile lined brick is ideal for anthracite and better then a stainless liner by a wide margin. The beauty of anthracite (Unlike wood or pellets) is that it makes no creosote thus you never need to have the chimney swept. All we ever do is take a shop vac and suck up any wayward fly ash that makes it's way around the bottom of thimble or into the stove pipe. If it's never been used it should be in excellent shape not compromised and the only thing I would check is that there is no birds nests or the like. If it's clear and you can see the light at the top with a hand mirror, you'll be good to go.
If you can see a square patch on the sheetrock then it's quite probable that you do have a thimble in there. If so when you open it, stick a small hand mirror in and look up the flue. If you see "the light"
PS: Ok just saw your flue and it looks fine. Now see if you have that thimble in back of the patch. You could use a wide chisel and hammer to cut round the patch also in case the bricks are close. (save the sawzall blade).