I found one years ago in a warehouse in about the same condition.. Ended up having a bidding war in my driveway between a couple pin collectors that came to look at it.. Rare pin!
Playfield does look pretty good for its age and how rare. You will pay through the nose for the backglass... About 8-10 years ago they sold a bunch of NOS glass off. So people who want it, probably already have it..
My local arcade had both joust and defender pins. I played them and black knight all the time back in the day, so they were must haves for me.. I was pretty lucky to find both of them.
Not sure yet, I have to go over everything before plugging it in.
Damn shame about the glass, but being stored with the head face down for so many years in a barn can't have been healthy