Drop-in resolution
Miyota 8217 vs Seiko NH38 (TMI NH38A): do they swap?
Whether the Miyota 8217 and Seiko NH38 (TMI NH38A) share a case, and what changes if you move between them.
Different footprints. The Miyota 8217 is 26.00 mm x 5.67 mm and the Seiko NH38 (TMI NH38A) 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 8217 is the 8200-family member adding a 24-hour indicator alongside the date. The Seiko NH38 (TMI NH38A) is the open-heart, no-date member of the NH3x family, in skeleton-dial mod watches.
They differ in diameter (26.00 mm vs 27.40 mm), height (5.67 mm vs 5.32 mm), calendar layout (centre seconds, date at 3, 24h sub-hand vs centre seconds, open heart, no date), winding behaviour. An 8200-family caliber at 26.00 mm x 5.67 mm that adds a 24-hour sub-hand to the 8215's date layout. It shares the family footprint but needs a dial with the 24h sub-register.
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.