One example exists, it is platinum, and the model it belongs to was built in steel for a reason. This Sonnerie Souveraine is a unique piece, made in a platinum case at 40 mm with a dial of 18K yellow gold and whitened guilloche silver and blued steel hands, and it now sits in a private collection. The calibre is the 1505, hand wound, built in 18K gold throughout. Hours and minutes at 3 o'clock, small seconds at 6, a power reserve indication, a strike mode selector, and the hammers themselves visible through an opening in the dial. Grande sonnerie, petite sonnerie, a silence position and an on-demand minute repeater, all of it running from a single mainspring. There is no second one, and the production model it derives from was discontinued in 2019.
Putting a sonnerie in platinum is a decision that costs something, and knowing what it costs is the point of this watch. F.P. Journe built the production Sonnerie Souveraine in steel and said plainly why he had done it: steel carries sound better than precious metal does, and on a watch whose entire purpose is to be heard, tone has to outrank material. Platinum is denser, softer and a good deal more absorbent acoustically than steel is. A platinum case will not ring the way a steel one does, and every watchmaker who has ever built a chiming watch in a precious metal has quietly accepted that trade in exchange for the metal itself and what it signals. Journe refused to make that trade at all for the production run, and said so publicly rather than quietly. Making one in platinum anyway, once, is therefore not a variant of the model. It is a deliberate exception to the principle the model was built on, and it only really makes sense as a one-off, for somebody who wanted the object rather than the optimum. Which is a perfectly legitimate thing to want. A grande sonnerie in platinum is still a grande sonnerie, and the difference in tone is a matter of degree rather than of kind.
The hammers are visible through the dial, and on a sonnerie that is a considerably more useful window than it first sounds. A grande sonnerie is running continuously whether you happen to be attending to it or not: it strikes the hour and the quarter every fifteen minutes, and the great majority of that activity happens while the watch is on a wrist under a sleeve, unwatched and very often unheard as well. Opening the dial over the hammers turns the striking from something that merely occurs into something the owner can actually inspect at any moment. It also tells you which mode the watch is currently in rather more reliably than the selector position does, since a hammer that lifts and never falls is a watch sitting in silence. The manufacture describes the concept of viewing the hammers through the dial as one of the more difficult parts of the entire development, which is entirely credible: a dial has to be cut away without weakening the plate that carries everything else, and the mechanism behind it has to be finished to be looked at rather than merely to function correctly. Very few chiming watches of any period show you anything at all from the front, and the ones that do rarely show you the part that moves.
Unique pieces occupy an odd position in this catalog, and it is worth being precise about what they actually are and are not. F.P. Journe has made a fair number of them over the years, mostly for charity galas and for particular clients, and broadly they divide into two kinds. Some are ordinary references carrying an extraordinary dial: engraved, enameled, or colored in some way the model never otherwise wore anywhere in production. Others, and this is plainly one of them, change something structural about the watch itself. A platinum case on a watch that was designed around steel is not a decorative variation at all; it alters the acoustic behavior of the object itself, which on a sonnerie is not a detail but the entire substance of the thing. That makes it a genuinely singular object rather than a special order placed against an existing menu, and it also makes it considerably harder to evaluate, because there is no second example anywhere to hear it against and no production standard against which the tone can be measured. Everything about it has to be taken on its own terms, which is either the appeal of a unique piece or the difficulty with one, depending on the buyer.
A unique grande sonnerie is bought on its documentation and on expert inspection, and neither of those is optional here. Establish the provenance completely: who commissioned it, when it was made, for what occasion if any, and whether the manufacture will confirm it, because on a one-off the paperwork is not supporting evidence, it is the primary record of what the object even is. Have the movement examined by somebody who has actually worked on a calibre 1505 before, of whom there are very few anywhere. Listen to it at length and across a full hour, understanding that the tone will not match that of a steel example and that this is entirely expected rather than a fault to be corrected. Cycle the selector through each of its positions in turn and watch the hammers respond through the dial. Confirm the yellow gold dial and its whitened guilloche silver are original and unrestored, since on a unique piece there is no reference specification to restore anything toward. Check the platinum case for the edge softening that repeated polishing produces, and treat any of it as permanent.
