/usr/lib/os-probes/mounted/70hurd is in os-prober 1.74ubuntu1.
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 14 15 16 | #!/bin/sh
set -e
. /usr/share/os-prober/common.sh
partition="$1"
dir="$2"
type="$3"
if [ -d "$dir/servers" ] && [ -d "$dir/hurd" ]; then
        label="$(count_next_label Hurd)"
	result "$partition:GNU/Hurd:$label:hurd"
	exit 0
else
	exit 1
fi
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