The only difference that I know of is that the old ones have an adjustable tree that is adjusted on a special machine with heat. The new ones have exchangeable gullet plates. Maybe there are more differences, but I don't know.
The only thing the government needs to solve all of its problems is a Council of Common Sense.
While the measurement that elleng may be true, keep in mind that a) there is a XW plate for the new model, and b) that the measurements are not absolute in terms of fit - much depends on the "footprint" of the tree - a shorter tree will fit differently/wider than a longer one with the same measurement.
Both models are great saddles and I personally love the flexibility of the exchangeable plates. The XW plate fits my elephant of a mare just fine - and she is so broad, with a very sloping shoulder and a short back, that she is often a hard fit.