Drop-in resolution
Seiko NH70 (TMI NH70A) vs Seiko NH72 (TMI NH72A): do they swap?
Whether the Seiko NH70 (TMI NH70A) and Seiko NH72 (TMI NH72A) share a case, and what changes if you move between them.
Same footprint, different calendar. The Seiko NH70 (TMI NH70A) and Seiko NH72 (TMI NH72A) share the 27.40 mm x 5.32 mm case fit, but the Seiko NH70 (TMI NH70A) is full skeleton, no date, no seconds hand and the Seiko NH72 (TMI NH72A) is full skeleton, no date, hacking, so a swap needs a dial cut for the new layout.
The Seiko NH70 (TMI NH70A) is the fully skeletonised member of the NH3x family, in open-worked mod watches. The Seiko NH72 (TMI NH72A) is the hacking, hand-winding skeleton member of the NH3x family.
They differ in calendar layout (full skeleton, no date, no seconds hand vs full skeleton, no date, hacking), 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.