Cal. 6620 (and the similar Cal. 6621) were quartz movements produced by Rolex between 1987 and 2015.
Rolex was part of the consortium that created the Swiss Beta 21 movement, which Rolex sold as Cal. 5100 in the limited-production Date 5100 watch. But even while supporting the Centre Électronique Horloger, Rolex began investing in their own movement operation. The resulting Cal. 5035 and Cal. 5055 (day/date) were launched in 1977 in the Rolex Oysterquartz watch. These would remain in production until 2003 with no direct replacement.
Rolex developed a similar movement for ladies watches. Cal. 6035 measured 19.79 mm in diameter and 5.00 mm thick, the same dimensions as their automatic movement. Although some prototypes were built, this movement never reached series production.
Rolex created a time-only movement with no seconds hand, Cal. 6620, for the Cellini line. Based on Cal. 6035, this new Cal. 6620 finally reached production in 1987. It was a slim movement, measuring 19.80 mm diameter by just 2.5 mm thick. The similar Cal. 6621 was introduced in 1990. Together, over 100,000 examples of Cal. 6620 and 6621 were produced before Rolex exited the quartz watch market after 2015.
Rolex also produced a model with a seconds hand, Cal. 6623.
Cal. 6620 features 8 jewels and operates at 32 KHz. It uses a stepper motor that operates once every 10 seconds.