The ID has changed. The ID was 44:6 but now thats nether slab. If you want the stone slab type: /give [yourname] 43 [amount] 7 . If you want the slab, then the ID would be: 44:7. If you want the block then this would be 43:7 . Its different from the original stone slab. Stacking the original stone slab has a line on the sides. Have fun with the undiscovered block. :)