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.