Drop-in resolution
Miyota 8205 vs Miyota 821A: do they swap?
Whether the Miyota 8205 and Miyota 821A share a case, and what changes if you move between them.
Same footprint, different calendar. The Miyota 8205 and Miyota 821A share the 26.00 mm x 5.67 mm case fit, but the Miyota 8205 is centre seconds, day and date at 3 and the Miyota 821A is centre seconds, date at 3, so a swap needs a dial cut for the new layout.
The Miyota 8205 is the day-date member of the 8200 family, with a weekday disc alongside the date. The Miyota 821A is the decorated 8215, with Cotes de Geneve and perlage on the same mechanism.
They differ in calendar layout (centre seconds, day and date at 3 vs centre seconds, date at 3). 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.