Drop-in resolution
ETA 2000-1 vs Sellita SW100-1: do they swap?
Whether the ETA 2000-1 and Sellita SW100-1 share a case, and what changes if you move between them.
Different footprints. The ETA 2000-1 is 19.40 mm x 3.60 mm and the Sellita SW100-1 is 20.00 mm x 3.35 mm, so they do not share a case. This is a choice between platforms, not a swap.
The ETA 2000-1 is a slim small automatic with date, common in ladies and mid-size watches. The Sellita SW100-1 is the Sellita small automatic, the alternative to the ETA 2000-1 in small cases.
They differ in diameter (19.40 mm vs 20.00 mm), height (3.60 mm vs 3.35 mm). A slim small automatic at 19.40 mm x 3.60 mm, thinner than the 2681. It shares the 2681's 19.40 mm diameter but at a lower height, so it is its own platform rather than a drop-in for the taller 2681.
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.