Drop-in resolution
Seiko NH70 (TMI NH70A) vs Seiko 6R15: do they swap?
Whether the Seiko NH70 (TMI NH70A) and Seiko 6R15 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko NH70 (TMI NH70A) (27.40 mm x 5.32 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 NH70 (TMI NH70A) is the fully skeletonised member of the NH3x family, in open-worked mod watches. The Seiko 6R15 is the predecessor of the 6R35, a date automatic in earlier Prospex and Presage models.
They differ in height (5.32 mm vs 5.25 mm), calendar layout (full skeleton, no date, no seconds hand vs centre seconds, date at 3), winding behaviour. A fully skeletonised NH3x caliber at 27.40 mm x 5.32 mm, sharing the family footprint and stem but designed to be seen through the dial. It needs a skeleton dial and has no seconds hand or date; the NH72 is its hacking, hand-winding sibling.
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.