Drop-in resolution
Sellita SW200-1 vs ETA 2801-2: do they swap?
Whether the Sellita SW200-1 and ETA 2801-2 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Sellita SW200-1 (25.60 mm x 4.60 mm) and ETA 2801-2 (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 Sellita SW200-1 is the primary Swiss alternative to the 2824-2, now the default in many microbrands. The ETA 2801-2 is the hand-wound, time-only member of the 2800 family, in slim dress watches.
They differ in height (4.60 mm vs 3.35 mm), calendar layout (centre seconds, date at 3 vs centre seconds, no date), winding behaviour. Matches the 2824-2 to the tenth of a millimetre in diameter and height, with the same date-at-3 layout, so it is the standard drop-in donor when a 2824-2 is unavailable. It carries one extra jewel (26 vs 25).
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.