Drop-in resolution
Seiko 7S26 vs Seiko 6R35: do they swap?
Whether the Seiko 7S26 and Seiko 6R35 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko 7S26 (27.40 mm x 4.90 mm) and Seiko 6R35 (27.40 mm x 5.25 mm) are near in size but differ in height, dial feet or layout, so moving between them means a new dial, hands and often a case adjustment.
The Seiko 7S26 is the long-running Seiko workhorse behind the SKX and Seiko 5, day-date, non-hacking. The Seiko 6R35 is the long-reserve Seiko date automatic, in higher-end Prospex and Presage models.
They differ in height (4.90 mm vs 5.25 mm), calendar layout (centre seconds, day and date at 3 vs centre seconds, date at 3), winding behaviour. The predecessor of the NH36 at 27.40 mm x 4.90 mm, without hand-winding or hacking. It shares the 27.40 mm case fit of the NH3x family, so an NH36 is the standard upgrade donor into a 7S26 case.
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.