Minecraft wiki entry for Metadata_variants, associated text: In 13w04a, double stone slabs were changed so that those with data values 8 through 15 would use the top texture on all six sides. This resulted in the return of the block that would several years later become smooth stone, and also introduced the smooth sandstone and smooth quartz blocks. However, as the smooth stone double slab block also housed the cobblestone, old (later petrified) oak, bricks, stone bricks and nether bricks slabs, these would also end up having "seamless" double slab variants that were visually no different from their normal double slabs or indeed their normal blocks. These blocks persisted up to 17w47a, where they were removed by the Flattening.