Developer and Support Tech
Yes, the extension determines where the original data is restored. But there's more to it than that.
1. Normally it's entirely useless to restore an old MBR to a new disk unless the new disk is exactly the same as the old disk, both in geometry as in how the disk is partitioned. It's not like: restore the MBR to a new disk and everything is back to the way it was before. Good chance you understand this but I'm mentioning it just in case.
2. The contents of the backup file also contain disk information, so restoring to another disk is tricky. This is by design: MBRtool is meant to backup the MBR and restore it to the same disk in case of trouble. It's not meant to be used in migrations, unless the original disk is cloned and therefore exactly the same as the new disk.
So, if you want to restore the backup anyway you'll have to edit the file and rename it first.