minecraft-assets/data/minecraft/recipes/anvil.json

{
  "type": "minecraft:crafting_shaped",
  "pattern": [
    "III",
    " i ",
    "iii"
  ],
  "key": {
    "I": {
      "item": "minecraft:iron_block"
    },
    "i": {
      "item": "minecraft:iron_ingot"
    }
  },
  "result": {
    "item": "minecraft:anvil"
  }
}