Drop-in resolution
Miyota 8215 vs Seiko 4R35: do they swap?
Whether the Miyota 8215 and Seiko 4R35 share a case, and what changes if you move between them.
Different footprints. The Miyota 8215 is 26.00 mm x 5.67 mm and the Seiko 4R35 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 8215 is the low-cost Japanese workhorse, common in entry-level and fashion automatics. The Seiko 4R35 is the branded Seiko date automatic behind many Presage and Prospex models.
They differ in diameter (26.00 mm vs 27.40 mm), height (5.67 mm vs 5.32 mm), winding behaviour. Close to the Swiss 11 1/2 core in diameter (26.00 mm) but taller at 5.67 mm and with a different dial-foot pattern, so it is not a drop-in for an ETA 2824 dial. Its seconds hand also has a soft, non-hacking stutter that is characteristic of the caliber.
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.