Vintage Aquadive find

Never heard of it myself, but do keep in mind the vintage Omega divers like the SM300 have a very different fit and feel from any ETA diver I've worn. The big difference is that most ETA divers, particularly those with sapphire crystals try to keep the crystal flush with the bezel and the bezel flush with the mid-case, which means everything else, hands, dial and movement sits lower in the case than you'd expect. Its why people with 2531.80 Bond Seamasters often complain that the crown digs into their wrist, because everything "inside" of the watch sits very low, while the mid-case, lugs and bezel sits up higher than the caseback.

The SM300 is the opposite of that, the bezel sits 5mm above the mid case and the crystal pokes high above that, giving it room for the movement, dial and hands to sit several mm higher than they would otherwise. The midcase isn't flat at all, its an arch that raises about 5mm up in the middle, so when its on your wrist, the case actually wraps around, instead of sitting flat, and everything inside it is offset upwards.

Hard to explain but you can see the difference in the picture and how it goes from low at the lug ends to high in the middle and how high the crown sits, almost touching the bezel, makes it the most comfortable watch I've ever owned, especially on mesh.

Interesting... yea it has a similar curve, just a bit of a thick caseback