The Mercedes hand was introduced on the 1953 Rolex Submariner ref. 6204. The reference also introduced large applied lume indices, the unidirectional rotating bezel, and the screw-down crown; the Mercedes hand was part of the same design package. The hour hand is a wider, lume-filled trapezoid with a circular tip; the circle is divided into three segments by metal cross-bars, with each segment filled with luminous compound. The visual effect at viewing distance: a three-pointed star inscribed in a circle.
Why the design: at depth (10m+ underwater) or in dim light, simple stick / sword / dauphine hands can be hard to distinguish from each other. The Mercedes hour hand is visually distinct from the minute hand (typically a sword or stick shape), so the diver can quickly read elapsed-bezel-time vs current-time without confusion. The three-segment lume also gives the hand more luminous area than a single-segment design, improving low-light readability.
"The Mercedes hand is one of the few design patterns where you can identify the brand from across the room. Rolex never put the crown on the dial. They didn't have to. The hand was the brand."- Watch-design commentary on the Mercedes hand
The name 'Mercedes' is collector slang, not Rolex marketing. The hand resembles the three-pointed Mercedes-Benz logo (which is a star within a circle); the watch-collector community started calling the design 'Mercedes hands' in the 1980s-90s. Rolex officially calls them 'hour hands' or 'Mercedes hands' inconsistently across documentation; the term 'Mercedes' has become shorthand within the watch-collector community.
Modern Mercedes hands are Super-LumiNova or Chromalight (Rolex's proprietary blue-glow lume). Vintage Submariner Mercedes hands often have tritium patina that ages to cream / yellow / brown over decades; the Mercedes shape with tritium is one of the most-recognised vintage-Rolex aesthetic markers. The design is essentially unchanged since 1953, though proportions have shifted slightly (the hand became slightly thicker on modern Submariner / GMT-Master II references). It remains a Rolex-only design pattern; competitors avoid it for both legal and aesthetic distinctness.