/sbin/mountro is in flashybrid 0.17.
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 | #!/bin/sh
#
# Remount the system in RO mode.
# Licensed under the terms GPL V2.
#
# - Diego Iastrubni <diego.iastrubni@xorcom.com>
mount -o remount,ro /
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