The Mk III now has patented composite stators instead of perforated metal, with a lifetime guarantee on the panels. The stators look different as, instead of holes, they have slots (of about 2mm wide). Based on the photos, they look even more transparent than the original perforated metal stators.
Wood trim (3 types inc. Cherry) is now a no cost option and a cloth grill is used over the woofer instead of perforated metal.
Not sure if there are any other changes - there could well be minor tweeks. Price has increased
to US$6,995, mainly due to the cost of the composite stators.