Drop-in resolution
Seiko 6R35 vs Miyota 8217: do they swap?
Whether the Seiko 6R35 and Miyota 8217 share a case, and what changes if you move between them.
Different footprints. The Seiko 6R35 is 27.40 mm x 5.25 mm and the Miyota 8217 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 6R35 is the long-reserve Seiko date automatic, in higher-end Prospex and Presage models. The Miyota 8217 is the 8200-family member adding a 24-hour indicator alongside the date.
They differ in diameter (27.40 mm vs 26.00 mm), height (5.25 mm vs 5.67 mm), calendar layout (centre seconds, date at 3 vs centre seconds, date at 3, 24h sub-hand), winding behaviour. A 27.40 mm date automatic with a 70 h reserve, slightly slimmer at 5.25 mm than the 4R35. It sits in the same 27.40 mm case family but is a Seiko-only caliber, not sold unbranded as an NH.
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.