The story I heard was that Gillette sued Browning for the use of the name Ultra because they were releasing an Ultra razor at that time and had already secured rights to the name "Ultra". When Browning was made aware of it they just droped the Ultra from their product as part of the law suit. I don't think there is any difference in the earilier Ultra and the later XT.
There is no difference in the Ultra XT and the XT. The Ultra's were the earlier XT's but browning had to drop the Ultra due to some legal issues regarding the use of Ultra in the name. I think it was due to the fact that another firearm manufacture used the term Ultra in the name of one of their firearms so browning could not for what ever reason.
I thought that the Ultra was a XS and that was was a patented name of onother company and that is why we now have the XT model. Same gun, different name. Sort of like Chinese resturants. Everytime the Board of health cites them for bugs or something, they change the resturant name.