I finally got started working on my new to me '91 851. When I pulled the belt cover something fell on the floor. Turns out the bike had lost a lock nut along the way and it got mixed up in the cam belt/pulley.
It was pretty easy to find where the nut came from.
I haven't pulled the belt yet to examine it and the pulley but I'm amazed the belt didn't let go when it was chewing up the nut. I guess those guys running open belts aren't as crazy as I thought?