The subject of rumour since Baselworld 2019, the Patek Philippe Grandmaster Chime ref. 6300A-010 in stainless steel is finally official.

Almost certainly on track to become the most expensive watch sold at auction in 2019, the ref. 6300A is a one of a kind watch variant of the most complicated Patek Philippe wristwatch ever made that was created for Only Watch, the biennial charity auction of unique timepieces sold to benefit a medical charity.

Aside from the steel case – the Grandmaster Chime is ordinarily available only in white gold – both dials on each side of the watch are also unique.

The dial on the front is made of 18k rose gold, and features a hobnail guilloche centre and blackened 18k gold Breguet numerals. “The Only One” is printed within the 12 o’clock sub-dial just to make it clear that this is, well, the only one for Only Watch.

Also made of 18k gold, the reverse dial is black with rose gold-plated hands.

The rest of the watch is identical to the standard version. The enormous 47.7mm case is reversible, but being in steel will be significantly lighter than the gold model.

The case band is decorated with a hobnail guilloche engraved by a hand-operated rose engine

Inside is the cal. 300 GS AL 36-750 QIS FUS IRM, a hand-wound movement with 20 complications, including a grande and petite sonnerie, alarm, date repeater, and instantaneous perpetual calendar.

Slated to go on the block on November 9, 2019 at Christie’s Geneva, the Grandmaster Chime “Only Watch” has an estimate of 2.0-2.5m Swiss francs, but on an average day it should comfortably top 7.0m Swiss francs, or even 10m francs on a great day.

That will include not just the watch, but lunch with Patek Philippe president Thierry Stern, and a visit to the company factory as well as its museum.

The rest of the Only Watch 2019 line-up can be found on onlywatch.com.