The Rolex 15xx series powered the Date, the Datejust, the GMT-Master and many other references from the early 1960s into the late 1970s. The numbers look simple, but the marking on the movement does not always match the name of the calibre. This guide explains how the family fits together, using examples from a Date ref. 1501, a Datejust ref. 1601 and a GMT-Master ref. 1675.
Know the family
The numbers describe different functions within one family. Cal. 1560 and 1570 are time-only movements. Cal. 1565 and 1575 add a date. The GMT versions add a 24-hour hand to the date calibre. The movement shown is a date movement whose bridge is marked 1570.
1570 or 1575? Reading the bridge
This is the point that confuses many buyers. A date-equipped Cal. 1575 can carry a bridge stamped "1570". Rolex used the same bridge across the family, so the number engraved on it does not identify the full calibre on its own. To know what you have, look at the functions the movement actually performs.
Two generations: the change in beat rate
The 1560/1565 family runs at 18,000 vibrations per hour. The later 1570/1575 family runs at 19,800 vibrations per hour. Date and GMT variants follow their respective families, so the beat rate is a useful way to tell the two generations apart.
Reference by reference: Date and Datejust
The Oyster Perpetual Date ref. 1501 used Cal. 1565 in the early 1960s and Cal. 1575 from about 1965 into the 1970s. The Datejust refs. 1600, 1601 and 1603 used Cal. 1565 until about 1965 and Cal. 1575 from about 1965 to 1977. The Turn-O-Graph ref. 1625, also known as the Thunderbird, used Cal. 1565 or 1575 through the 1960s and 1970s. These dates are approximate and production overlapped.
The GMT version: ref. 1675
The GMT-Master ref. 1675 followed the same pattern. Early production, up to around 1965/66, used Cal. 1565 GMT; from about 1965/66 it used Cal. 1575 GMT. GMT adds a 24-hour hand to the date calibre. Some of the earliest examples marked "Officially Certified Chronometer" use Cal. 1535. The watch shown is a 1675/8 in yellow gold.
Three functions: date, quickset and hacking
Three details describe how these movements behave in daily use. The date change is instant: it jumps around midnight. There is no quickset date on the 1565 and 1575; the date is set by advancing the hands. Stop seconds, or hacking, was added to later 1570-series movements in the early 1970s, so early and late examples of the same calibre differ here.
Read the whole watch
One engraving is not the whole story. The case reference sits between the lugs, here "1675" under "Registered Design". The movement and its functions tell you the calibre. Confirm the reference, the era, the functions and the movement together, and the picture becomes clear.
Rolex in our collection
We offer vintage Rolex Date, Datejust and GMT-Master references from this period, each checked and serviced before sale. The watches currently available are on our Rolex page, and you are welcome to see the movements in person at our Omotesando store.
