Good question - I am going to take a stab at it and say that the new model has the cross bolt safety and the old does not. I'm quite sure there are others here that will be able to give you a definate answer, so let's see what they say.
woody, I think you'll need to order from what they call the "late model". Take a look at the schematics on their web site and look at the lever and lower tang on both the early and late models and choose accordingly. One of my 39A's is a '57 model and I always order for the "late" model. The cross-bolt safety doesn't have anything to do with it as neither the early or the late models have them.