3005. Rolex. A very fine 18K gold automatic calendar wristwatch Rolex. A very fine 18K (750) yellow gold wristwatch with date automatic movement Model: Datejust 'Thunderbird', Second series Reference: 6309 (First Generation 1953-1959) Date: Cira 1955 Movement: 18-jewel automatic, No.99744 Dial: Champagne stained, applied gilt dart hour markers, gilt outer 1/5th second divisions with luminous dot 5 minute markers, magnified red and black date aperture at 3, gilt dagger hands with luminous inserts, centre seconds Case: Brushed and polished tonneau form, screw down Oyster back and crown, bi-directional calibrated 'Turn-O-Graph' bezel, No.141401 Strap/Bracelet: Brown leather Buckle/Clasp: Steel buckle Signed: Case & movement Size: 36mm Weight: 55,31g The inaugural reference 6202 introduced the rotating bezel with 60 unit markers, and at the time, the bezel was made of Bakelite. This model was crafted in stainless steel and equipped with a glossy black dial and a gilt (gold plated) chapter ring. Rolex followed it up with a few units of the reference 6202 in stainless steel and yellow gold, which was really ahead of its time. It was very quickly discontinued, and only a handful of models are known. The second series - reference 6309 and 6609 - introduced an upgraded movement, and amped up the dressy aesthetic with solid gold editions. They were produced from 1954 to 1959. The third series - reference 1625 - officially made the Turn-o-Graph a part of the Datejust collection. The "Turn-o-Graph" text was removed from the dial, leaving only "Datejust" and adding "Superlative Chronometer Officially Certified". Two main variations of the 1625 were made available - both in stainless steel, with a choice between a white gold engine turned bezel, or a yellow gold bezel with matching centre links. Several bracelet options were also offered, from the Jubilee and Oyster, to leather straps. While the collection is named the Turn-o-Graph in Rolex literature, it was also marketed as the "Thunderbird" in North America. By the end of the 1950s, Rolex supplied the US Air Force's aerobatic squadron (named Thunderbird) with Turn-O-Graph watches, as the rotating bezels proved handy in navigating flight calculations.