Another potential option, @MichaelD65300 , if you're not finding an email that was only sent to List B when restoring it, would be to make a segment. The segment would need to have the criteria of:
Contact Activity > Were Sent > Specific Email > the email you'd sent to that list
AND
List Membership > On list > your List B
Once that segment is calculated, you can use it to restore List B after removing all the List A contacts, by adding the segment to List B.
If the change to your contacts was done within the last 24 hours, you could call our general support to confirm security permissions and petition our Tier 2 team to do a contact rewind. However there's no guarantee that the rewind will be entirely successful or a complete reversal of the accidental merge. Even still, it's usually the method that requires the least effort for your sake.
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