postinst is in libpam-script 1.1.6-1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | #!/bin/sh
# postinst script for libpam-script
#
# see: dh_installdeb(1)
set -e
pam-auth-update --package
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