Drop-in resolution
Seiko 6R35 vs Seiko 6R15: do they swap?
Whether the Seiko 6R35 and Seiko 6R15 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko 6R35 (27.40 mm x 5.25 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 6R35 is the long-reserve Seiko date automatic, in higher-end Prospex and Presage models. The Seiko 6R15 is the predecessor of the 6R35, a date automatic in earlier Prospex and Presage models.
Their published figures are the same to the tenth of a millimetre. 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.