Drop-in resolution
ETA 2804-2 vs Miyota 821A: do they swap?
Whether the ETA 2804-2 and Miyota 821A share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The ETA 2804-2 (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 2804-2 is the hand-wound date member of the 2800 family, the manual sibling of the 2824-2. 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)), winding behaviour. The manual-wind, date-at-3 version of the 2800 family at 25.60 mm x 3.35 mm. It adds a date disc to the 2801-2, so it needs a dial with a date window and swaps with the Sellita SW215-1.
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.