Normal wood slabs have the ID 126, but the items you are showing actually have the ID 44:2. 126 is associated with wooden slabs, but 44 belongs to stone slabs (for example, cobble slabs are 44:3).

"Wooden Slab"s were officially replaced with "Oak Wood Slab"s in 1.3.1, if I recall correctly. However, existing slabs were not converted. I don't know where you found that slab, but it dates back to 2012. They are actually still pretty common (especially in old towns), I have a few stacks of them as well.


The TL;DR: Those slabs were Minecraft's prototypes for wooden slabs in 2011 and 2012 and are essentially stone slabs with a plank texture.