Drop-in resolution
Donor movement for the ETA 251.264
Which movement drops into a ETA 251.264 case, and what has to match beyond the footprint.
No mechanical drop-in. The ETA 251.264 is a quartz caliber; swapping it for an automatic of a similar diameter changes power source, height and hand heights, so it is a case redesign, not a drop-in.
The ETA 251.264 is a battery quartz caliber at 25.60 mm x 5.50 mm. Replacing it like-for-like means another quartz movement of the same family and footprint, not an ETA, Sellita, Miyota or Seiko automatic.
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.