minecraft-assets/assets/minecraft/models/block/polished_grimstone_stairs_inner.json

{
  "parent": "minecraft:block/inner_stairs",
  "textures": {
    "bottom": "minecraft:block/polished_grimstone",
    "top": "minecraft:block/polished_grimstone",
    "side": "minecraft:block/polished_grimstone"
  }
}