minecraft-assets/assets/minecraft/blockstates/smooth_sandstone_slab.json

{
    "variants": {
        "type=bottom": { "model": "block/smooth_sandstone_slab" },
        "type=top": { "model": "block/smooth_sandstone_slab_top" },
        "type=double": { "model": "block/smooth_sandstone" }
    }
}