I think that NT headstamp indicates "Non Toxic" ammo loaded with non toxic components. I am just guessing, but perhaps Winchester only has small primers with non toxic compound available for plant use when they make those . I have some cases like that I just picked up off the range floor last night and they also have the small primer feature. The NT ammo also has no exposed lead on the bullet bases. Federal also markets NT ammo. The exclusive use of this kind of ammo may be coming for all indoor ranges; that day has already arrived for some of them. Coincidentally, it causes shooters to buy and use only the range's ammo.
PMC sells a line of NT primers, but they cost more than regular and are not seen for sale commonly.