The new style, on the left - at least I have never seen it before - is smaller and the post mount is closer to the dish, as it appears! the neck is also shorter 5 1/8 inch vs 6 1/8 inch. The posts are the same size, however. I think you may see it in the picture, that he pivot point is closer in the smaller mount and the pivot bolt runs through the mount so the mount is not open, like the older style. It looks like it could easily be turned upside down, though. I do not have a motor to try it on to see how it would work.
All the arms for the 75E's are the same... The 85E is different but I do have it by itself!