Drop-in resolution
Miyota 9039 vs Seiko NH35 (TMI NH35A): do they swap?
Whether the Miyota 9039 and Seiko NH35 (TMI NH35A) 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 NH35 (TMI NH35A) 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 NH35 (TMI NH35A) is the default modding and microbrand automatic, hacking and hand-winding at low cost.
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.