Called Park Tools today and here's the answer: The four bolts that attach the caliper arm to the uprights were eliminated in the 4.2 in favour of welding. Apparently the bolts got in the way of very wide tires.

First time calling Park Tools directly and they were a pleasure to deal with. Answered on the second ring and got transferred to a very knowledgeable technical rep. It's such a rarity to get that kind of service these days.