HI Dave,
i have FT team kit and it only came with the bolt and a silver locknut, the locknut sits into a hexagonal shape section inside the plastic outdrive.
IRS said to read there instruction but the part number they refered to was the same as the bolt and nut, no TNUT. I winder if it was on one of the parts tree as a spare, will have to look.
other wise thatnks for the part number
regards brad
Originally posted by DaveW
OHBRAD: If the "T-nut" is still spinning, odds are you cut the ears off of it. They sit in the slots of the outdrive to keep it from spinning. You can get the bolt and the nut together in ASC6575, as you already mentioned, or in a complete rebuild kit for the diff in ASC3926. The diff rings included in ASC3926 will fit the IRS outdrives as well as the TC3 steel outdrives. Double check your white "T-nut" and let me know if you accidentally cut the ears off of it. Im hoping it is as simple as that. 
- Dave