Drop-in resolution
Miyota 9039 vs Seiko 4R35: do they swap?
Whether the Miyota 9039 and Seiko 4R35 share a case, and what changes if you move between them.
Different footprints. The Miyota 9039 is 26.00 mm x 3.90 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 9039 is the no-date member of the 90 series, popular in clean-dial microbrand watches. 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 (3.90 mm vs 5.32 mm), beat rate (28,800 A/h (4 Hz) vs 21,600 A/h (3 Hz)), calendar layout (centre seconds, no date vs centre seconds, date at 3), winding behaviour. The no-date sibling of the 9015 at 26.00 mm x 3.90 mm, sharing the same dial-foot location. It drops into a 9015 case as long as the dial has no date window, which is its whole point.
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.