/usr/share/initramfs-tools/hooks/rescuevol is in cloud-initramfs-rescuevol 0.27ubuntu1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 | #!/bin/sh
set -e
PREREQS=""
case $1 in
prereqs) echo "${PREREQS}"; exit 0;;
esac
. /usr/share/initramfs-tools/hook-functions
copy_exec /sbin/blkid /sbin
# vi: ts=4 noexpandtab
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