Since the 1940s, Aqua-Lung has been an unforgettable name in SCUBA diving circles. It was developed by the patron saint of SCUBA divers himself, Jacques Cousteau. With a pedigree like that, it’s no surprise that anything with the unmistakeable logo—meant to resemble a tank of compressed air with twin hoses typical of Cousteau’s Aqua-Lung design—would be desired by collectors.

Especially watches.

There’s no mistaking it: the advent of the Aqua-Lung self contained underwater breathing apparatus (or SCUBA) made skin-diving as a sport much more accessible to lovers of the sea. By 1946 the Aqua-Lung was being sold in France to commercial and amateur divers, and by the 1950s it was available in the US and Great Britain. Its portability and the relative ease it brought to breathing underwater allowed more and more people to pursue the sport.

Soon Cousteau formed a company, U.S. Divers, for distribution of Aqua-Lung and other branded merchandise.

Alongside fins, masks, and regulators, U.S. Divers also sold Aqua-Lung branded watches. Though perhaps best-known for their co-branded Blancpain and DOXA dive watches, they also sold watches with the Aqua-Lung logo. Using the same A. Schild contract case that was used by brands from Blancpain on down, these watches also used the sturdy, no-nonsense A. Schild/Felsa Calibre 4004 movement found in Blancpain’s acclaimed Fifty Fathoms.

None of these watches was a chronograph, however… but this one is.

At face value, this chronograph—known as 666 for the last three digits on the case back—has much in common with its time-only brethren. It has the same contract case, capped by a black rotating bezel. But the dial holds the secret to what beats inside: a Landeron Calibre 248 chronograph movement, beloved by many in the industry.

Very few of these chronographs exist, or have survived more or less intact, making them relatively rare. Though exact quantities are unknown, experts believe that the watch was only produced for one year. Some suppose that it might have been released as a trial run that never went to full production.

Ours has been treated to a recent spa treatment (including having the dial and hands relumed). The case retains the same sharp lugs and hefty proportions that made the A. Schild contract case so desirable, surmounted by the now handsomely-weathered bezel. Uncommon and undeniably attractive, it offers the collector a rarely-seen diving chronograph with a sterling diving pedigree.