The 15400ST was the 41 mm steel Royal Oak with date from 2012 to 2019. It succeeded the 39 mm 15300ST and took the Royal Oak Selfwinding to 41 mm in steel for the first time, a size that has remained the standard since. The dial has a Grande Tapisserie pattern and appeared in black, white and blue, among other colours.
The in-house calibre 3120 has a power reserve of about 60 hours and a 22 ct gold rotor, visible through the display back. The watch is water-resistant to 50 metres. In 2019 the 15400ST was succeeded by the 15500ST, with the larger calibre 4302 and a slightly revised dial.
On a pre-owned example we look first at the finishing: a Royal Oak that has been polished several times loses the sharp edges and clear transitions between brushed and polished surfaces that carry the design. We also check the certificate and set, the folding clasp and the date of the last service, because calibre 3120, like any mechanical movement, needs periodic maintenance. A smaller counterpart is the 37 mm 15450ST.