Drop-in resolution
Miyota 2115 vs Seiko VH31 (SII): do they swap?
Whether the Miyota 2115 and Seiko VH31 (SII) share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Miyota 2115 (23.30 mm x 3.60 mm) and Seiko VH31 (SII) (23.30 mm x 3.45 mm) are near in size but differ in height, dial feet or layout, so moving between them means a new dial, hands and often a case adjustment.
The Miyota 2115 is a widely used Japanese three-hand date quartz, in mid-range quartz watches. The Seiko VH31 (SII) is the sweeping-seconds quartz caliber that mimics a mechanical sweep at low cost.
They differ in height (3.60 mm vs 3.45 mm), calendar layout (centre seconds, date at 3 vs centre sweeping seconds, no date, hacking), winding behaviour. A three-hand date quartz caliber at 23.30 mm x 3.60 mm, a step up in size from the 2035. It is its own quartz footprint and not a mechanical donor for any automatic.
Matching diameter and height is necessary but not sufficient. A donor is only a true drop-in when the dial-foot positions, hand-hole sizes, stem length and calendar layout also match. Two movements can share a case and still need a new dial, re-broached hands or a cut-to-length stem.