Drop-in resolution
ETA 2801-2 vs Miyota 821A: do they swap?
Whether the ETA 2801-2 and Miyota 821A share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The ETA 2801-2 (25.60 mm x 3.35 mm) and Miyota 821A (26.00 mm x 5.67 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 ETA 2801-2 is the hand-wound, time-only member of the 2800 family, in slim dress watches. The Miyota 821A is the decorated 8215, with Cotes de Geneve and perlage on the same mechanism.
They differ in diameter (25.60 mm vs 26.00 mm), height (3.35 mm vs 5.67 mm), beat rate (28,800 A/h (4 Hz) vs 21,600 A/h (3 Hz)), calendar layout (centre seconds, no date vs centre seconds, date at 3), winding behaviour. The manual, no-date base of the 2800 family at 25.60 mm x 3.35 mm. It shares the family diameter but has no rotor and no calendar, so it swaps only with the Sellita SW210-1 hand-wound caliber.
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.