You don't get to say one of one very often, but that's very likely what you're looking at. This is an early Favre-Leuba Bathy 50, the first wirstwatch to feature a mechanical depth gauge. It's already a fascinating corner of early tool watches in that respect alone. But all other Mark 1 (ref. 53243) Bathy 50s you'll find which have been photographed and documented have black dials. This, the only example now known and very likely a prototype, is blue, originally from a long-standing retired Favre-Leuba employee in the 1960s.

Now, if you know Favre-Leuba, you'll know that the latter Mark 2 (ref. 53253) Bathy 50s featured a blue dial. You may think someone overly fond of navy moved some dials around. But the two are not interchangeable; the sizes are completely off. We've been lucky enough to get them side by side, and even some small details like the spacing of the Swiss Made are unique to the Mark 1, the blue here is darker as well.

And the Mark 1 is already a very interesting proposition, most notably as the first to sport a mechanical depth gauge through a very complex compressible inner caseback. You can just barely see its workings if you peer through the recessed, open caseback grips. But then also for it's Heuer-like 40mm case with a wide bakelite bezel, playful handset, and 'flat logo' caseback. The Mark 1 was available with a dial in metres or feet, this example coming from Switzerland is characteristically in metres. Inside is a manual Peseux 320, widely lauded as one of the most dependable and beautiful movements of its time. For context, Kari Voutilainen's early Observatoire work utilized a very heavily-reworked and highly-finished but similar Peseux 260. Despite the bulk to accommodate the sophisticated caseback tech, it’s surprisingly wearable and gives off instant playful summer attitude.

The Bathy 50 never sold in volume, despite the innovation it is estimated that fewer than 1000 examples were ever produced. When you're up against Rolex advertising, it's hard to sell. It's a recipe that never really caught on, but is worth revisiting again today purely out of love for the genre. That, and the fact that this dial hasn't been captured before.

This example presents in excellent vintage condition. The case maintains defined bevels, with light to moderate surface wear commensurate with age. No significant bashes, just wear. There is one scratch pictured on caseback deeper than the rest. Its dial shows no faults, damage, or strikes of any kind. The scales and print text are very clear and minimally aged. The handset tritium has gone a bit warm grey in age, matching the dial. Its bakelite bezel has no cracks, with lovely cream in the luminous portions. Bezel, dial, and hand tritium match perfectly under UV. It is still sporting its original, very domed bezel. The movement has been serviced recently and is running well in spec. One of the fun quirks of this design is that it activates in an airplane's pressurized cabin, which is what happened before photography here. The shift out of cabin pressure was slight enough that it is still reading a few metres down. A quick service could reset this if necessary, but perhaps a flight would as well. I wouldn't personally choose to dive with it anymore to reset it, it has not been pressure tested recently. It comes on the pictured navy tropic rubber, which brings out this rare dial beautifully.

This watch is modeled on a relatively large 7.7 inch wrist.

• Brand: Favre-Leuba
• Model: Bathy 50
• Reference: 53243
• Size: 40
• Year: c. 1965
• Case Material: Steel
• Movement: Manual Peseux Calibre 320
• Scope: Watch Only