Drop-in resolution
ETA 251.264 vs Seiko VK64 (SII): do they swap?
Whether the ETA 251.264 and Seiko VK64 (SII) share a case, and what changes if you move between them.
Different footprints. The ETA 251.264 is 30.00 mm x 5.00 mm and the Seiko VK64 (SII) is 30.80 mm x 5.10 mm, so they do not share a case. This is a choice between platforms, not a swap.
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The ETA 251.264 is a Swiss quartz chronograph caliber, in analogue quartz chronographs. The Seiko VK64 (SII) is the bicompax meca-quartz chronograph, in two-register chronograph mods.
They differ in diameter (30.00 mm vs 30.80 mm), height (5.00 mm vs 5.10 mm), calendar layout (chronograph, multi-subdial with date vs meca-quartz bicompax chronograph, two sub-dials, date), winding behaviour. A thermocompensated quartz chronograph at 30.00 mm x 5.00 mm (13 1/4 ligne). It shares its diameter with the 7750 family but not its 7.90 mm height or its power source.
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.