Fill-Side Behavioral Concentration on Polymarket: Identification Limits under Record-Level Attribution

This paper studies behavioral concentration in Polymarket's public executed-fill record and formalizes what that record can and cannot identify. A pre-publication reconciliation corrects the empirical scope: the archived extraction covers the legacy CTF Exchange over Polygon blocks 86,008,447-86,107,178, approximately 25 April 2026 17:09 UTC through 28 April 2026 00:00 UTC, rather than the full 21-27 April week stated previously. It contains 13,356,931 OrderFilled records, 77,204 addresses with at least five attributed records, and 43,116 token identifiers; negative-risk markets are absent. The archived feature construction credits both maker and taker addresses on each record. This convention is not invariant to match fragmentation, and mint/burn executions do not admit a universal buyer/seller interpretation. The reported one-cluster result is therefore retained only as a null under the original record-level representation, not as evidence that the participant population is intrinsically unimodal. The concentration table is likewise attribution-weighted arithmetic under that convention. Two methodological results remain durable: public fills do not identify the quote lifecycle required to infer market making, spoofing, or strategic withdrawal; and a one-cluster result rejects density separation in an observed feature space, not latent economic heterogeneity. A match-normalized, mint-aware, multi-window replication is required before treating the cluster null or exact cohort shares as stable venue properties.