Drop-in resolution
ETA 2671 vs ETA 2681: do they swap?
Whether the ETA 2671 and ETA 2681 share a case, and what changes if you move between them.
Different footprints. The ETA 2671 is 17.20 mm x 4.80 mm and the ETA 2681 is 19.40 mm x 4.80 mm, so they do not share a case. This is a choice between platforms, not a swap.
The ETA 2671 is the small ladies automatic, in compact and slim cases with a date. The ETA 2681 is the slightly larger small ladies automatic, a size up from the 2671.
They differ in diameter (17.20 mm vs 19.40 mm). A miniaturised automatic at 17.20 mm x 4.80 mm, effectively a shrunken 2824. It shares nothing dimensionally with the 11 1/2 ligne core: it is its own small footprint, paired only with the slightly larger 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.