postinst is in original-awk 2012-12-20-1.
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 | #!/bin/sh
set -e
update-alternatives --quiet --install /usr/bin/awk awk /usr/bin/original-awk 0 \
--slave /usr/share/man/man1/awk.1.gz awk.1.gz /usr/share/man/man1/original-awk.1.gz
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