The 15202ST is the 39 mm Royal Oak Extra-Thin, known to enthusiasts as the 'Jumbo'. It is the closest to the original Royal Oak of 1972, reference 5402ST: the same size, a case about 8 mm thick, no seconds hand, a date at three o'clock and a Petite Tapisserie dial.
Calibre 2121 is one of the thinnest self-winding movements with a date and goes back to the movement fitted to the first Royal Oak in 1972. It has a power reserve of about 40 hours and runs at a lower frequency than modern calibres, which is part of this movement's character. The watch is water-resistant to 50 metres. The 15202ST was made from 2012 to 2022; in 2022 the 16202ST followed with the new calibre 7121.
Within that production run there are several versions of dial and bracelet, and the full reference on the certificate shows which one it is. On a pre-owned example we look at the dial, at the finishing of the thin case and the bracelet, and at the set. With a watch that may be more than ten years old, its service history also matters.