Drop-in resolution
Seiko NH36 (TMI NH36A) vs Seiko 6R15: do they swap?
Whether the Seiko NH36 (TMI NH36A) and Seiko 6R15 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko NH36 (TMI NH36A) (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 NH36 (TMI NH36A) is the day-date sibling of the NH35, adding a weekday disc on the same footprint. 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, day and date at 3 vs centre seconds, date at 3). Shares the NH35's 27.40 mm x 5.32 mm footprint and stem, so it drops into an NH35 case. The only change is the added day disc, which needs a dial with a day window and correct printing. It is the unbranded 4R36.
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.