.SYNCDB File

The syncdb file extension is used by iTunes for saved contacts list in database format.

Found as backup.syncdb, these files are created during the backup process of iDevice in iTunes.

These files are typically located:

macOS (OS X)

/Library/Application Support/MobileSync/Backup folder.

Microsoft Windows

C:\Users\"user name"\AppData\Roaming\Apple Computer\MobileSync\Backup

How to open:

You can import these databases back to iTunes should you need it.

How to convert:

Possibly contacts can be exported and saved in a separate file, like VCF.