Drop-in resolution
Miyota 821A vs Seiko 6R35: do they swap?
Whether the Miyota 821A and Seiko 6R35 share a case, and what changes if you move between them.
Different footprints. The Miyota 821A is 26.00 mm x 5.67 mm and the Seiko 6R35 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 821A is the decorated 8215, with Cotes de Geneve and perlage on the same mechanism. The Seiko 6R35 is the long-reserve Seiko date automatic, in higher-end Prospex and Presage models.
They differ in diameter (26.00 mm vs 27.40 mm), height (5.67 mm vs 5.25 mm), winding behaviour. Mechanically and dimensionally the 8215 at 26.00 mm x 5.67 mm, dressed up with finishing. It is a genuine drop-in for an 8215 case: the footprint, stem and dial feet are identical.
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.