Drop-in resolution
Seiko 4R35 vs Seiko 6R35: do they swap?
Whether the Seiko 4R35 and Seiko 6R35 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko 4R35 (27.40 mm x 5.32 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 4R35 is the branded Seiko date automatic behind many Presage and Prospex models. The Seiko 6R35 is the long-reserve Seiko date automatic, in higher-end Prospex and Presage models.
They differ in height (5.32 mm vs 5.25 mm). The branded Seiko caliber that the NH35 copies, at 27.40 mm x 5.32 mm with date at 3. It shares the NH35 footprint and stem, so the two interchange within the 27.40 mm 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.