5 kg Eisen

The "5 kg Eisen" — five kilograms of iron — is a commodity-backed Kriegsgeld (war money) voucher, one of dozens of material-value notes issued by German municipalities and industrial firms during the First World War when metal coinage vanished from circulation and confidence in abstract paper currency was low enough that issuers anchored denominations to tangible raw materials: iron, coal, rye, electricity by the kilowatt-hour.

Without issuer details in the data, precise attribution is impossible — but the format itself is the story: a denomination expressed in weight of pig iron rather than Marks or Pfennig.