Drop-in resolution
Seiko NH72 (TMI NH72A) vs Seiko 6R15: do they swap?
Whether the Seiko NH72 (TMI NH72A) and Seiko 6R15 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko NH72 (TMI NH72A) (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 NH72 (TMI NH72A) is the hacking, hand-winding skeleton member of the NH3x family. 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, hacking vs centre seconds, date at 3). The hacking, hand-winding skeleton member of the NH3x family at 27.40 mm x 5.32 mm. It shares the NH70 footprint and stem but adds hand-winding and hacking, and like the NH70 it needs an open-worked dial.
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.