If you were to goto a junkyard and remove this spoiler from an actual outback, there would be 3 bolts inside the hatch you would undo, and then you could remove the spoiler.
The 3 bolts goto studs that are fitted to the spoiler. On older Legacy's, these studs point right at the rear glass. To make it mountable, I removed the studs, and the self adhesive from the spoiler and sanded it smooth so it would fit flush against the hatch.
You could mount it. You would need to use an epoxy or some sort of adhesive. If your 97 did not originally come with a spoiler, this may make it easier to install.