Drop-in resolution
Miyota 8205 vs Seiko 4R36: do they swap?
Whether the Miyota 8205 and Seiko 4R36 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 4R36 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 8205 is the day-date member of the 8200 family, with a weekday disc alongside the date. The Seiko 4R36 is the branded day-date Seiko automatic, the retail version of the NH36.
They differ in diameter (26.00 mm vs 27.40 mm), height (5.67 mm vs 5.32 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.