The Challenger trans is similar to the one in the Genesis and several AGCO brands, but not identical.  I think what I was told is that the changes were necessary to get the lower gears to hold up with the traction of belts.   The US Genesis were set up to function as 16 speed like the Challengers but the AGCOs had gearing to use all 18 gears as unique speeds...the other two had overlapping gears in both high and low range to cover the middle gears so theoretically you would not have to do the double pack shift under load.