Due to how the slabs are implemented (a pretty direct copy-paste from vanilla slabs) they cannot easily be implemented in a mod in 1.12.
The blocks expect the "base" slab block to be there when creating the upper and lower slab, which isn't the case in 1.12. They have to go through the ForgeRegistry events first.
We could maybe try something with "@ObjectHolder".

The recipe json's for these slabs are in "src/main/resources/assets/appliedenergistics2/disabledrecipes/".

• Minecraft Version: 1.12
• AE2 Version: rv5-alpha-0
• Forge Version:

should be fixed?