www.communitycurrencieslaw.org

Anchor text: 8 U.S.C. § 486 – Uttering Coins of Gold, Silver or Other Metal: "Whoever, except as authorized by law, makes or utters or passes, or attempts to utter or pass, any coins of gold or silver or other metal, or alloys of metals, intended for use as current money, whether in the resemblance of coins of the United States or of foreign countries, or of original design, shall be fined under this title or imprisoned not more than five years, or both." Note that 18 U.S.C. § 491 also addresses the creation of coins, but this particular code section prohibits the creation of coins or the use of similar metal objects for the purpose of inserting into parking meters, vending machines, and similar venues.

Wikipedia: Slug (coin)

Wikipedia: Non-monetary economy