The Day Date became Rolex's flagship. The first model (6511) lasted just over a year before it was replaced by the externally identical 6611 with caliber 1055 movement with its free sprung Micro-Stella balance and was the first to be labeled "Superlative Chronometer Officially Certified" on the dial. This new accuracy standard guaranteed the watch to remain within + 3.0 seconds per day. This is the official birth of the Rolex Legend model Day Date… I have been longing to see one.
Today, I can have one in my hand, it is original Ref 6611. The Rolex Day Date, the fluted bezel has a finer pitch, the hour Marks is a thick piece of wedge shaped sold gold set on the dial, there is “"Superlative Chronometer Officially Certified” on the dial at the six o’clock position.
This model has been released for about two years only, there should not too many example of good condition Ref 6611. In 1959 caliber 1055 was replaced with caliber 1555 in model 1803 (18,000bph) and later In 1965 - model 1803 received caliber 1556 (19,800bph). In 1977 caliber 3055 (quick-set) was introduced in model 18038.
In 1988 caliber 3155 (double quick-set) was introduced in model 18238.
Caliber 3155 is the caliber still used today in all Day-Date models.