Rolex Day-Date :

The Basel fair, the new Day-Date model (model 6511) had a window at the 12 position spelling out the day of the week and the numbered date was shown at the 3 position. Initially introduced as a niche model, The first model (6511) lasted just over a year before it was replaced by 6611. This model featured the new and improved caliber 1055 movement with its free sprung Micro-Stella balance and was the first to be labeled "Superlative Chronometer Officially Certified". This statement guaranteed the watch to remain within + 3.0 seconds per day. With very little design changes over the years, the first modification to the Day-Date was the addition of the "hack" feature in 1972. This allowed the second hand to be stopped dead while the winding stem was completely pulled out, allowing the time to be synchronized with an accurate source. The Day-Date was available in 18kg gold or platinum.