Drop-in resolution
Seiko 7S36 vs Seiko 6R15: do they swap?
Whether the Seiko 7S36 and Seiko 6R15 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko 7S36 (27.40 mm x 5.60 mm) and Seiko 6R15 (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 7S36 is the extra-jewel sibling of the 7S26, in many Seiko 5 sports models. The Seiko 6R15 is the predecessor of the 6R35, a date automatic in earlier Prospex and Presage models.
They differ in height (5.60 mm vs 5.25 mm), calendar layout (centre seconds, day and date at 3 vs centre seconds, date at 3), winding behaviour. A day-date automatic at 27.40 mm x 5.60 mm, the 7S26 with two more jewels and extra bracing. It shares the 27.40 mm NH3x case fit, so an NH36 is the usual hacking upgrade donor into a 7S36 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.