NVG261 is a heavier duty t-case (higher torque rating). It is the case that is used in my '01 2500HD. I am not sure about the bolt pattern on the adapter or the spline count that is used. My guess would be that they are TH400/4L80e 6 bolt pattern with a 32 spline input, since they use them behind 4L80es as well as the Allison 1000 like in my truck, but until I have a reason to remove mine from the truck, it will remain a mystery. The 261 also uses a magnesium case, which saves weight but otherwise is not a plus in my book (corrosion!).