Drop-in resolution
Seiko 4R35 vs Miyota 8218: do they swap?
Whether the Seiko 4R35 and Miyota 8218 share a case, and what changes if you move between them.
Different footprints. The Seiko 4R35 is 27.40 mm x 5.32 mm and the Miyota 8218 is 26.00 mm x 5.67 mm, so they do not share a case. This is a choice between platforms, not a swap.
The Seiko 4R35 is the branded Seiko date automatic behind many Presage and Prospex models. The Miyota 8218 is a day-date 8200 member, in entry automatics that show the weekday.
They differ in diameter (27.40 mm vs 26.00 mm), height (5.32 mm vs 5.67 mm), calendar layout (centre seconds, date at 3 vs centre seconds, day and date at 3), winding behaviour. The branded Seiko caliber that the NH35 copies, at 27.40 mm x 5.32 mm with date at 3. It shares the NH35 footprint and stem, so the two interchange within the 27.40 mm case.
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.