Minecraft wiki entry for Seamless_Stone_Slab, associated text: Slabs 0-7 in Beta 1.6. There are four seamless stone slabs.
Slabs 0-7 in Beta 1.8. There are two seamless stone slabs.
Slabs 0-7 in 1.4.6. There is one seamless stone slab.
Slabs 0-7 in 1.5. There are no seamless stone slabs.