I know that there is a way to do it in the Mindcrack pack. Ethoslab on YT created a lot, but didn't show the recipe.
the old smoothstone slabs that looked sort-of like marble.(as in, light-colored stone, not the marble from RP2) the double slab was two stacked on top of each other to form a full block.
EDIT for the sake of clearing something up: in the current version of the both the Mindcrack DW pack stone slabs are craftable from three smoothstone, just as they were before. I don't see what the complaint is about.
...They removed the default minecraft clean double slab from the game when they added nether brick slabs. As the double nether brick slab now occupies the data value of the old double clean slab.
I'm pretty sure the seamless slabs used to be 43:6, which is where Nether Brick slabs are now. It's the only way to explain how the seamless slabs Etho had all turned into Nether Brick double slabs.
I think that was Advanced Solars.Does anyone know what mod it was in the beta pack that gave that block a crafting recipe?