Drop-in resolution
Seiko 6R35 vs Seiko NH70 (TMI NH70A): do they swap?
Whether the Seiko 6R35 and Seiko NH70 (TMI NH70A) 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 NH70 (TMI NH70A) (27.40 mm x 5.32 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 NH70 (TMI NH70A) is the fully skeletonised member of the NH3x family, in open-worked mod watches.
They differ in height (5.25 mm vs 5.32 mm), calendar layout (centre seconds, date at 3 vs full skeleton, no date, no seconds 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.