Drop-in resolution
Miyota 821A vs Seiko NH35 (TMI NH35A): do they swap?
Whether the Miyota 821A and Seiko NH35 (TMI NH35A) share a case, and what changes if you move between them.
Different footprints. The Miyota 821A is 26.00 mm x 5.67 mm and the Seiko NH35 (TMI NH35A) is 27.40 mm x 5.32 mm, so they do not share a case. This is a choice between platforms, not a swap.
The Miyota 821A is the decorated 8215, with Cotes de Geneve and perlage on the same mechanism. The Seiko NH35 (TMI NH35A) is the default modding and microbrand automatic, hacking and hand-winding at low cost.
They differ in diameter (26.00 mm vs 27.40 mm), height (5.67 mm vs 5.32 mm), winding behaviour. Mechanically and dimensionally the 8215 at 26.00 mm x 5.67 mm, dressed up with finishing. It is a genuine drop-in for an 8215 case: the footprint, stem and dial feet are identical.
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.