Master Chronometer certification is not an evolution of COSC: it is a distinct protocol developed by Omega together with METAS (Swiss Federal Institute of Metrology) and introduced in 2015 with the Globemaster. The fundamental difference lies in what is being tested. COSC certifies the raw movement, not yet assembled. METAS certifies the finished watch, with case, crown and crystal: what arrives on the customer's wrist.
There are three parameters. First, precision: between 0 and +5 seconds per day, measured in six positions, at two temperatures and at two power reserve levels. COSC accepts -4 to +6 seconds, only on the bare movement. Second, magnetic resistance: at least 15,000 gauss, equivalent to the field of a hospital MRI machine. Third, water resistance: the depth declared by the manufacturer is verified on the certified piece, not left to declaration alone. In total, METAS performs eight tests over ten days.
Tudor has adopted this protocol on its in-house calibers, marking them with the U suffix in the reference number — MT5601-U, MT5602-U, MT5652-U and so on — and has built a TRC (Tudor Reliability Control) department within the manufacture with robots operating continuously to replicate and document these conditions. The fact that Tudor, traditionally associated with a more accessible positioning than Omega, chose the same standard is significant: it indicates that certification is establishing itself as an industry reference, not as a marketing tool for a single brand.
One clarification worth making: some sources confuse Master Chronometer with COSC, attributing the acronym to the Geneva body alone. This is not the case. COSC remains a preparatory phase: the movement passes it first, then — if the manufacturer chooses — it undergoes the METAS process. The letter U in the Tudor code and the Master Chronometer inscription on an Omega dial both indicate that this second passage has occurred and been documented.
I cannot say, however, how much the actual production tolerances deviate from the limits declared in real batches: those data are not public and I have never had access to aggregate statistics from METAS laboratories.