In contract manufacturing the customer has one real guarantee: that the delivered part matches the technical drawing exactly. That conformity can only be demonstrated through measurements taken at every stage of production and the reports handed over with the delivery.
A chain of traceability has to run from incoming raw material all the way to final inspection, and critical dimensions must be verified with calibrated instruments. What we have learned since 1976 is simple: quality is not a coincidence, it is a system.