The shutter has definitely gone to Canon heaven.
It's dropped a blade and what you are seeing is the blade across the image.
The circles are where the rivets have torn away.
Over the years, I've done three in various bodies.
If you lift up the mirror - you won't do any damage - and you'll see it.
Here's what my 1D MkII looked like
Sorry, missed the post where you had figured it out.
Anyway, someone might find this useful.