What is the name of Rolex's in-house certification that exceeds COSC?

Rolex’s in-house certification that exceeds COSC is called Superlative Chronometer.

The designation applies to a fully assembled Rolex watch after its movement has passed the independent Swiss Official Chronometer Testing Institute, or COSC. COSC tests the uncased movement; Rolex then performs its own final controls on the cased watch, testing performance in conditions intended to represent actual wear.

Rolex redefined the certification in 2015. Under the modern standard, the finished watch is expected to meet a precision tolerance of approximately −2/+2 seconds per day, substantially tighter than COSC’s mechanical chronometer range of −4/+6 seconds per day. The broader Rolex protocol also addresses power reserve, waterproofness and self-winding performance.

“Chronometer” alone is too general: it describes a precision watch or movement meeting an official standard. “Master Chronometer” is a separate designation most closely associated with METAS testing, particularly for Omega watches, and includes magnetic-resistance requirements. Rolex’s Superlative Chronometer status is symbolized by its green seal and accompanied by a five-year international guarantee.