/lib/partman/active_partition/25divider/choices is in ubiquity 2.18.7.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 | #!/bin/sh
# Not just a blank line because debconf would select that as the default
printf "divider\t%s\n" " "
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