This paper proposes that Indus Valley Civilization stamp seals (c. 2600–1900 BCE) functioned as institutional credit instruments — the structural equivalent of modern credit cards — within a guild-backed transaction system. The Indus civilization sustained long-distance trade across five million square kilometers without coinage. Building on a prior statistical analysis of 179 Mohenjo-Daro unicorn seal inscriptions (Kriger & Hunt 2026, doi:10.5281/zenodo.19103880), five convergent lines of evidence are presented: (1) near-total sequence uniqueness (98.3%, p < 0.001), analogous to unique account numbers; (2) positional entropy profiles consistent with formatted alphanumeric codes; (3) intaglio manufacture enabling impression transfer, functionally identical to embossed credit card imprinting; (4) centralized, tamper-resistant production paralleling institutional card issuance; and (5) archaeological find contexts consistent with a non-transferable credential guaranteeing creditworthiness. The paper reconstructs a five-stage transaction cycle (issuance, identification, recording, clearing, retirement), reinterprets duplicate seals as replacement issuances, and develops the international function of seals found at Mesopotamian sites as credit credentials. A new section on phonetic vocalization reconciles the credit-instrument hypothesis with Parpola's Proto-Dravidian readings: the codes were spoken aloud for communication but the vocalization was not what the codes encoded. The interpretation is situated within the broader scholarship on pre-monetary credit (Graeber 2011; Hudson 2018) and is compatible with Mukhopadhyay's (2023) administrative interpretation and the Decision Framework proposed in Kriger (2026). All claims are qualified to the Mohenjo-Daro unicorn subcorpus. Keywords: Indus script, credit instruments, pre-monetary economies, institutional credit, Indus seals, Bronze Age trade, cashless transactions, registration codes, Mohenjo-Daro, Harappan civilization Related identifiers: Kriger, B., & Hunt, T. A. (2026). Positional constraints, sequence uniqueness, and stroke numerals in Indus seal inscriptions from Mohenjo-Daro. doi:10.5281/zenodo.19103880 (is supplemented by this upload) License: CC BY 4.0