Drop-in resolution
Miyota 82S5 vs Seiko 6R15: do they swap?
Whether the Miyota 82S5 and Seiko 6R15 share a case, and what changes if you move between them.
Different footprints. The Miyota 82S5 is 26.00 mm x 5.67 mm and the Seiko 6R15 is 27.40 mm x 5.25 mm, so they do not share a case. This is a choice between platforms, not a swap.
The Miyota 82S5 is the small-seconds open-heart 8200 member, in decorated entry automatics. The Seiko 6R15 is the predecessor of the 6R35, a date automatic in earlier Prospex and Presage models.
They differ in diameter (26.00 mm vs 27.40 mm), height (5.67 mm vs 5.25 mm), calendar layout (small seconds, open heart, no date vs centre seconds, date at 3), winding behaviour. An 8200-family open-heart caliber at 26.00 mm x 5.67 mm with small seconds rather than the 82S0's central seconds. It shares the family footprint but needs its own small-seconds, open-heart dial.
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.