Seiko SX Quartz Ref. 5Y31-8A00 – February 1987 – flat bi-color case

Five and a half millimeters in height, 28 grams in weight, and a dial with almost nothing on it but dots — the Seiko SX is restraint as a design philosophy.

The case back is stamped with SEIKO, BASE METAL, ST. STEEL BACK, the reference 5Y31-8A00, and the serial number 720347. The dial is marked with SEIKO QUARTZ and the line designation SX.

The SX Line

The 1980s were the decade of the ultra-thin quartz watch in Japan. Once movement engineering had surpassed mechanical competition, the focus of the rivalry shifted to design—to height, dial aesthetics, and the question of how inconspicuous a watch can be without becoming invisible.

The SX was Seiko’s answer in the premium dress segment. The 5Y31 caliber is a simple, extremely thin quartz movement that makes the low profile possible in the first place. The result is a watch you barely feel on your wrist—and that was exactly its selling point.

Design

The champagne-colored dial does without a single index in the traditional sense. Instead, small hemispherical applications sit at the hour positions, framed by a fine dotted minute track. The result is a dial that appears almost empty from two meters away and is completely clear at reading distance.

The hands are extremely slim and long, while the seconds hand is a thin line. The lettering is limited to SEIKO QUARTZ above and SX below the center.

The case is two-tone: the flanks and the wide, curved lugs are lightly brushed, while the bezel ring and lug edges are gold-colored. This two-tone finish was the hallmark of the upscale dress watches of those years. The watch comes on a brown, crocodile-embossed leather strap.

Please note: The case back identifies the case as Base Metal with a stainless steel back.

Dating

The serial number 720347 dates the watch to February 1987 according to the standard Seiko system. The first digit indicates the year, the second the month. This corresponds to the known production period of the 5Y31 caliber.

Condition and Origin

The watch comes from a Japanese specialty watch shop and was inspected there prior to sale.

The dial is well-preserved; there are a few small dark spots in the area between 4 and 5 o'clock that are not noticeable at a normal viewing distance. All applications are secure, and the hands are complete and undamaged. The crystal is clear.

The case shows signs of wear and polishing; slight abrasion is visible on the edges of the gold-colored parts. The case back has fine scratches, but the stamping is fully legible. The leather strap has been worn but is supple and intact.

The actual condition is shown in the unretouched product photos.

Technical Data

• Brand: Seiko
• Model line: SX
• Reference: 5Y31-8A00
• Serial number: 720347
• Manufacturing date: February 1987
• Movement: Seiko Caliber 5Y31, quartz
• Functions: Hour, minute, second
• Case: Base metal with stainless steel back, two-tone
• Diameter without crown: 31.2 mm
• Height: 5.6 mm
• Lug-to-lug: 36.9 mm
• Lug width: 18 mm
• Weight incl. strap: 28 g
• Dial: champagne-colored with hemispherical applications
• Strap: brown crocodile-embossed leather strap
• Origin: Japan

For whom?

For those who want a watch that does not call attention to itself on the wrist. For slim wrists. And for collectors of Japanese dress quartz watches, for whom the case height is the decisive criterion.

Warranty (used)

This is a used item. The limitation period for claims for defects under the statutory warranty is 12 months from delivery.

Note on water resistance

Water resistance is not guaranteed. To protect the movement, the watch should not come into contact with water or moisture.