Drop-in resolution
Donor movement for the ETA 2681
Which movement drops into a ETA 2681 case, and what has to match beyond the footprint.
No same-family drop-in in the popular core. Fitting a different caliber to a ETA 2681 case means matching its 19.40 mm x 4.80 mm footprint and accepting a dial, hand and stem change.
The ETA 2681 does not have a like-for-like twin in this popular-donor set. The honest answer is to source the same caliber, or move to a different footprint and rebuild the dial, hands and stem around it.
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.