What a sweet find!
Is it possible that it was left out in part because the feed mechanism is broken? Or that, being out in the elements did some damage?
I can't help with the manual, but I think you're on the right track with eliminating user error first. From your description, it sounds as if there's an obstruction below the ribbon, or more likely that it's jammed and the system knows enough to not try to force the paper and is disengaging the feed. Can you manually move the print ribbon arms?