Wooden Slabs from versions before 1.3.1 don't have a model in the inventory

Sadly, Mojang has removed all non-default blocks.
Duplicate of MC-58445 (Works as Intended)

This isn't higher than default, or a custom damage value, though. It's a valid, used (in older maps), block. It's model is still in game (since it's just the normal wooden slabs), and it only appears incorrect when held in hand and in the hotbar.

Apologies, regarding the double stone slab, gone - MC-57793

"Some block types are only used internally and won't have an item anymore." - Searge

Ah, that makes more sense as a reason for this. The thing is, though, that the block in question is legitimately obtainable in survival, had you been playing prior to the update that these blocks were retired in. They're not converted to the new type either, so older worlds may still contain these blocks.

Invalid item.

@@unknown what do you mean by "Invalid item"?

Can confirm this in 1.8 at least.

@Pierre, it means the block has no purpose. Mojang won't fix it. Have to move on.

Basically, they don't support worlds made before 1.7 anymore. It's a simple big, though. Fixing it through a mod wouldn't be too hard, I assume.

I just don't see why they didn't just convert them in to oak wooden slabs in the first place...
It would be better to call them "Old unsupported items" than to cal it an "invalid item". Because since this was obtainable in survival on an unmodded server I don't think of them as invalid really.

Fixed in 17w47a