Drop-in resolution
Miyota 8217 vs Seiko NH34 (TMI NH34A): do they swap?
Whether the Miyota 8217 and Seiko NH34 (TMI NH34A) 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 NH34 (TMI NH34A) 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 NH34 (TMI NH34A) is the affordable modding GMT automatic, adding a caller-GMT hand to the NH35 base.
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, GMT 24h hand, date at 3), 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.