Drop-in resolution
ETA 2801-2 vs ETA 2804-2: do they swap?
Whether the ETA 2801-2 and ETA 2804-2 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 ETA 2804-2 (25.60 mm x 3.35 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 ETA 2804-2 is the hand-wound date member of the 2800 family, the manual sibling of the 2824-2.
They differ in calendar layout (centre seconds, no date vs centre seconds, date at 3). 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.