The Hammoburg guys do a lot of good work. They are basing their material on much of what Steve Hand wrote about between the relations between Renaissance shield sources, the duelling shields seen in German sources such as Talhoffer, and the period iconography of Viking and Migration art work. Since there are no known sources of sword and shield (besides bucklers) prior to the Renaissance, there is a fair bit of extrapolation from other areas to figure out what was historically done.
