Drop-in resolution
Miyota 8205 vs Seiko 7S26: do they swap?
Whether the Miyota 8205 and Seiko 7S26 share a case, and what changes if you move between them.
Different footprints. The Miyota 8205 is 26.00 mm x 5.67 mm and the Seiko 7S26 is 27.40 mm x 4.90 mm, so they do not share a case. This is a choice between platforms, not a swap.
The Miyota 8205 is the day-date member of the 8200 family, with a weekday disc alongside the date. The Seiko 7S26 is the long-running Seiko workhorse behind the SKX and Seiko 5, day-date, non-hacking.
They differ in diameter (26.00 mm vs 27.40 mm), height (5.67 mm vs 4.90 mm), winding behaviour. Shares the 8215 footprint at 26.00 mm x 5.67 mm but adds a day disc at 3, so it needs a dial with day and date windows. It swaps with an 8215 case only if the dial carries the extra 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.