Drop-in resolution
Miyota 8218 vs Seiko 6R15: do they swap?
Whether the Miyota 8218 and Seiko 6R15 share a case, and what changes if you move between them.
Different footprints. The Miyota 8218 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 8218 is a day-date 8200 member, in entry automatics that show the weekday. 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 (centre seconds, day and date at 3 vs centre seconds, date at 3), winding behaviour. A day-date 8200-family caliber at 26.00 mm x 5.67 mm, sharing the 8205's day-and-date layout and the family footprint. It drops into an 8200-series case with a matching day-date 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.