Drop-in resolution
Seiko VH31 (SII) vs Ronda 705: do they swap?
Whether the Seiko VH31 (SII) and Ronda 705 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko VH31 (SII) (23.30 mm x 3.45 mm) and Ronda 705 (23.30 mm case fit (23.90 mm overall) x 2.50 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.
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The Seiko VH31 (SII) is the sweeping-seconds quartz caliber that mimics a mechanical sweep at low cost. The Ronda 705 is a slim no-date Ronda quartz caliber at the 10 1/2 ligne size.
They differ in height (3.45 mm vs 2.50 mm), calendar layout (centre sweeping seconds, no date, hacking vs centre seconds, no date, slim), winding behaviour. A quartz caliber at 23.30 mm x 3.45 mm whose seconds hand sweeps in quad-pulse steps to imitate a mechanical movement. It is a compact quartz footprint of its own and not a mechanical donor.
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.