When ever I use a negtive number for the Skin ID, the json file gets overwritten and changed back to the default. (I am getting the ID numbers from here: https://umod.org/documentation/games/rust/definitions#items )

       "1. Shortname": "mining.pumpjack",
        "2. Chance": 75,
        "3. Minimal amount": 1,
        "4. Maximal amount": 1,
        "5. Skin ID": -1130709577,
        "6. Display name": "Pump Jack",
        "7. Blueprint": false

skin id can't be lower than 0. Its always postive value. You selected there item id instead of skin id. Just leave that filed as 0