Drop-in resolution
Seiko NH34 (TMI NH34A) vs Seiko 6R15: do they swap?
Whether the Seiko NH34 (TMI NH34A) and Seiko 6R15 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko NH34 (TMI NH34A) (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 NH34 (TMI NH34A) is the affordable modding GMT automatic, adding a caller-GMT hand to the NH35 base. 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 (centre seconds, GMT 24h hand, date at 3 vs centre seconds, date at 3). The GMT member of the NH3x family at 27.40 mm x 5.32 mm, adding a 24-hour hand to the NH35's date layout. It shares the NH35 case footprint and stem, so it drops into an NH35 case but needs a GMT dial and hand set. It is the unbranded Seiko 4R34.
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.