Why hide the part that beats?

An open-heart dial is a compromise between a plain dial and a full skeleton. One aperture is cut where the balance wheel sits, and the rest of the dial is left alone. You get to watch the escapement without giving up legibility.

Seiko builds this into its Presage dress line at a few hundred dollars. The same idea on a Swiss dial usually costs several times as much, which is the reason the Presage open hearts get recommended so often.

The movement is from Seiko’s 4R family, the in-house automatics that run at 21,600 vibrations per hour with a stated reserve of about 41 hours. They hack and they hand-wind. Seiko’s SSA425 page lists the 4R39; our catalogue has long listed the 4R38, and both are open-heart calibers with a subdial.

What the subdial shows depends on the exact reference, and that is where this page is careful. See the note below.

Seiko describes SSA425 as a black dial on a steel bracelet with a 24-hour subdial. The photograph on this page shows a cream dial on a brown leather strap with a subdial marked 0, 20 and 41, which is how Seiko labels a power-reserve indicator.

They are very likely two different Presage open-heart references. Before you buy, match the dial colour, strap and subdial on the listing to the one you actually want. The core specification, a 4R open-heart automatic under box Hardlex with 50m of water resistance, is common to both.