Drop-in resolution
Miyota 90S5 vs Seiko 4R35: do they swap?
Whether the Miyota 90S5 and Seiko 4R35 share a case, and what changes if you move between them.
Different footprints. The Miyota 90S5 is 26.00 mm x 3.90 mm and the Seiko 4R35 is 27.40 mm x 5.32 mm, so they do not share a case. This is a choice between platforms, not a swap.
The Miyota 90S5 is the open-heart member of the 90 series, showing the balance through the dial. The Seiko 4R35 is the branded Seiko date automatic behind many Presage and Prospex models.
They differ in diameter (26.00 mm vs 27.40 mm), height (3.90 mm vs 5.32 mm), beat rate (28,800 A/h (4 Hz) vs 21,600 A/h (3 Hz)), calendar layout (open heart, no date vs centre seconds, date at 3), winding behaviour. A 90 series caliber at 26.00 mm x 3.90 mm with an open-heart aperture that exposes the balance. It shares the 9015 footprint and dial feet but needs a dial cut for the open-heart window.
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.